This set me a-searching too, and here are some things I found.
In addition to the SOP itself, I've also looked at
The Spirit of Prophecy Treasure Chest, ©1960 The Ellen White Estate; a book that used to come with the Prophetic Guidance course from Voice of Prophecy, now out of print. And
Notes and Papers, from the EGW Estate and Andrews University, 7th printing (1974); a book that accompanies the seminary course on "Prophetic Guidance."
As far as Ellen White exhibiting physical phenomena while in vision in public, it appears that she did. While I couldn't find a specifically narrated account of a public vision with signs present, they are mentioned by those who were intimately familiar with Sis. White's work.
Arthur L. White, in the article "How the Visions Were Given" in the
Spirit of Prophecy Treasure Chest, p. 19, wrote:
"We might divide these manifestations into three general groups:
1. Visions given in public, and accompanied by the marked physical phenomena.
2. Visions given during the hours of the night, often referred to as the prophetic dreams.
3. Visions given during periods of prayer or writing, unaccompanied by physical phenomena."
The public visions with physical phenomena noted always had the instances of EGW not breathing for minutes or hours, even being examined by medical doctors; yet "at the same time her movements and gestures, which are frequent, are free and graceful, and cannot be hindered nor controlled by the strongest person," according to James White (
Life Incidents, p. 272).
George I. Butler, former GC President, wrote that
"For nearly thirty years these visions have been given with greater or less frequency, and have been witnessed by many, oftentimes by unbelievers as well as those believing them. . . .While she is in vision, her breathing entirely ceases. No breath ever escapes her nostrils or lips when in this condition. This has been proved by many witnesses, among them physicians of skill, and themselves unbelievers in the visions, on some occasions being appointed by a public congregation for the purpose. . .
"There are none of the disgusting grimaces or contortions which usually attend spiritualist mediums, but calm, dignified, and impressive, her very appearance strikes the beholder with reverence and solemnity. There is nothing fanatical in her appearance." -- Review & Herald, June 9, 1874. (quoted in Spirit of Prophecy Treasure Chest, p. 27)
J.N. Loughborough stated to the 1893 General Conference session:
"I have seen Sister White in vision about fifty times. . . .Her last open vision was in 1884, on the camp ground at Portland, Oregon. She has visions at the present time, but they are not open visions in a public assembly. It is a heavenly place to be in, where there is an open vision, as some of those here who have seen her at such times, can testify.
"The first indication that she is about to be taken off in vision is that she loses all strength, like a person suddenly falling down. This state continues not more than five seconds, when she suddenly rises to her feet. She herself says that the first thing she knows an angel stands by her side and touches her and she receives strength." -- General Conference Daily Bulletin, Jan. 29, 1893 (quoted in Notes and Papers, p. 74)
Evidently he'd seen her have "open visions in a public assembly", but by this time they had ceased.
***
While it is true that in the early Advent experience there were those who were genuinely prostrated before the Lord by His Spirit, we still have some warnings about the counterfeit:
"I saw that Satan was working through agents in a number of ways. He was at work through ministers who have rejected the truth and are given over to strong delusions to believe a lie that they might be damned. While they were preaching or praying, some would fall prostrate and helpless, not by the power of the Holy Ghost, but by the power of Satan breathed upon these agents, and through them to the people. While preaching, praying, or conversing, some professed Adventists who had rejected present truth used mesmerism to gain adherents, and the people would rejoice in this influence, for they thought it was the Holy Ghost. Some even that used it were so far in the darkness and deception of the devil that they thought it was the power of God, given them to exercise. They had made God altogether such a one as themselves and had valued His power as a thing of nought." EW 43
"Whenever and wherever the Lord works in giving a genuine blessing, a counterfeit is also revealed, in order to make of none effect the true work of God. Therefore we need to be exceedingly careful, and walk humbly before God, that we may have spiritual eyesalve that we may distinguish the working of the Holy Spirit of God from the working of that spirit that would bring in wild license and fanaticism. 'By their fruits ye shall know them.' Those who are really beholding Christ will be changed into His image, even by the Spirit of the Lord, and will grow up to the full stature of men and women in Christ Jesus. The Holy Spirit of God will inspire men with love and purity; and refinement will be manifest in their characters." RH 02-06-94
Lastly, she mentioned that
"In the future we shall have special tokens of the influence of the Spirit of God -- especially at times when our enemies are the strongest against us. The time will come when we shall see some strange things; but just in what way, -- whether similar to some of the experiences of the disciples after they received the Holy Spirit following the ascension of Christ, -- I can not say." MS 115, 1908
She spoke of "strange things" that will be coming from God! So it's evident that the "by their fruits ye shall know them" and "to the law and to the testimony" tests will be vital in discerning these things.
Like Thomas said, we can't let the counterfeit make us afraid of the real thing. If I gave the impression in the earlier post that I was saying that all 'prostrations of the Spirit' were of the devil, it was unintentional...apologies to all. It's just that the satanic variety appears to be so much more widespread these days -- we need to be careful.