I was puzzled whether these two strings, which certainly sound a little odd, couldn't nevertheless be correct? Even if 'state' is the more commonlly used word in such strings, 'mention' seems just as right.
As so stated
As so mentioned
And also, regarding the string:
As it was so stated
Or
As was so stated
Could you write these more simply as:
As so stated
Without overstretching the accepted bounds of deletion?
There are some examples out there of similar, though they are rare outside of legal contexts. None of them appear to mean as it was so... or as was so...; mostly they would be best reworded done in that/this way, for example in the following as so conceived could be reworded as conceived in this way.
The purpose of retribution is to succor the violated from injustice by requiring the violator to share the wound he or she has inflicted. As so conceived, retributive justice differs fundamentally from revenge with which critics of retribution confuse it; for what summons it into being is the victim's longing to be rescued from evil, not the desire of the victim to violate her violator. (Journal: Theological Studies; 2009; WOUND MADE FOUNTAIN: TOWARD A THEOLOGY OF REDEMPTION; Miller, Jerome A)
Another example
In his discussion of the resemblance of Poe's poetry to that of the Bible, Forrest uses a different term: "What is known as envelope structure is the use of an initial refrain repeated at the end of the poem instead of in the opening of its middle stanza. As so used it encloses the thought which should be read in the light of the refrain" (Journal: Style; 1999 (Winter); A Catalogue of Selected Rhetorical Devices Used in the Works of Edgar Allan Poe; Zimmerman, Brett)
The Corpus of US Supreme Court Opinions produces 60 hits at the start of a sentence - in order of highest to lowest frequency, As so: construed, interpreted, understood, modified, read, viewed, computed, defined, stated, written, weakened, regarded, planned, located, enacted, constructed, characterized, amended, administered. There are only two examples for as so stated, one right after the other in the same opinion:
This contention ignores the basic principle underlying common-law remedies that they shall afford only compensation for the injury suffered, Milwaukee, etc., R. R. Co. v. Arms et al., 91 U.S. 489 ; Chicago, etc., Ry. Co. v. McCaull-Dinsmore Co., supra, 100 of 253 U. S., 40 S. Ct. 504; Robinson v. Harman, 1 Exch. 850, 855; Sedgwick, Damages (9th Ed.) 25; Sutherland, Damages (4th Ed.) 12; Williston on Contracts, 1338, and leaves out of account the language of the amendment, which likewise gives only a right of recovery for 'actual loss.' The rule urged by respondents was applied below in literal accordance with its conventional statement. As so stated, when applied to cases as they usually arise, it is a convenient and accurate method of arriving at an amount of recovery which is [281 U.S. 57, 64] compensatory. As so stated, it would have been applicable here if there had been a failure to deliver the entire carload of coal, since the wholesale price, at which a full carload could have been procured at point of destination, would have afforded full compensation (ILLINOIS CENT. R. CO. v. CRAIL 281 U.S. 57 (1930))
As for the particular string as so mentioned, it is not unheard of, but again, mainly in legal contexts, and apparently meaning in the way mentioned earlier
An appeal against an invalidity notice must be initiated by serving a notice of disagreement upon the valuation officer and, unless the valuation officer withdraws the invalidity notice within four weeks of the service of the disagreement notice, the valuation officer must inform the clerk of the relevant valuation tribunal, on the expiry of that period: (a) of the entry in the list (if any) which it is proposed to alter; (b) of the grounds on which the proposal was made?; and (c) of the reasons for the valuation officer's opinion that the proposal has not been validly made. Where such information has been supplied and the invalidity notice is withdrawn, the valuation officer must inform the clerk of the valuation tribunal of the withdrawal as soon as is practicable. Until it is finally decided that the proposal to which the invalidity notice relates was validly made, no further steps for processing the proposal can be taken. However, where it is finally decided as so mentioned those procedures have effect as if the proposal had been served on the valuation officer on the date of that final decision. (Halsbury's LAWS OF ENGLAND 5TH EDITION)