I don't think Scott would be a plausible Know Nothing candidate in 1856 (and realistically, the remaining Whigs know they need a candidate with Know Nothing support). He had flirted with nativism in the mid-1840's but publicly repudiated it in 1848:
"WASHINGTON, May 29. 1848.
Dear Sir:-—In reply to your kind letter of the 8th inst. I take pleasure in stating that, grateful for the too partial estimate you place on my public services, you do me no more than justice in assuming that I entertain “kind and liberal views towards our naturalized citizens.” Certainly it would be impossible for me to recommend or support and measure intended to exclude them from a just and full participation in all civil and political rights now secured to them by our Republican laws and institutions.
It is true. that in a season of unusual excitement, years ago. when both parties complained of fraudulent practices in the naturalization of foreigners, and when there seemed to be danger that native and adopted citizens would be permanently arrayed against each other in hostile factions, I was inclined to concur in the opinion then avowed by leading statesmen, that some modification of the naturalization laws might be necessary, in order to prevent abuses, allay strife, and restore harmony between. the different classes of our people. But later experience and reflections have entirely removed this impression, and dissipated my apprehensions.
In my recent campaign in Mexico, a very large proportion of the men under my command were your countrymen, Irish, Germans, &c. I witnessed with admiration their zeal, fidelity, and valor in maintaining our flag in the face of every danger, vieing with each other and our native-born soldiers in the same ranks in patriotism. constancy, and heroic daring. I was happy to call them brothers in the field, as I shall always be to salute them as countrymen at hame. I remain, dear sir, with great esteem, yours truly,
WINFIELD SCOTT. WM. E. ROBINSON, Esq."
https://books.google.com/books?id=cxqcm9IV1eIC&pg=RA2-PA12
In 1852, he had been the choice of the "pro-Catholic" Seward for the Whig nomination, and conducted a campaign where "the Whigs planted friendly Irish questioners in audiences addressed by Scott, giving the candidate a chance to declare how much he 'loved to hear that rich Irish brogue.'"
https://books.google.com/books?id=BZIftd0Ip0IC&pg=PA102 (This just alienated nativists without gaining any appreciable number of Irish votes.
Even if Scott had not been nominated in 1852, so the "Irish brogue" comments were never made, nativists had reason to suspect him for his protection of Catholic property in Mexcio, for his 1848 Robinson letter, and perhaps for the fact that his daughter Virginia converted to Catholicism and became a nun at Visitation Convent in Georgetown.
I don't see any chance of his becoming a Republican, either. Even in 1860 he supported Bell (and was in fact mentioned as a possibility for the Consitutional Unionist Party nomination). Incidenrally, no less a southern Rights man than John Quitman (who had served under Scott in Mexico) was astonished that southern Whigs persuaded themselves that Scott was anti-slavery: "I have been surprised and astonished...that among the Whig party there should be found a single man unwilling to give him a cordial and hearty support. As to his being 'controlled by Seward'--that is mere stuff! I know the man, and he will be controlled by no one contrary to his own
convictions of what is right. And as to his being true to the South, I consider him the most unexceptionable man, on that score, among all the Whigs who have been named in connection with the presidency. I am a Democrat; and consequently, differing widely as I do from General Scott on every political question, can never give him my support; but if there is a Whig in the Union for whom, under any circumstances, I could cast my vote for President, that Whig is Winfield Scott!"
http://books.google.com/books?pg=PA174&id=H12mtLfdlQcC
I also don't think it's likely he will become a Democrat, but even if he did, with his long history of Whiggery, there is no way the Democrats are going to nominate him.