Well, I think Ult Steve is sensitive to mockery, and Ult Tony does tend to mock. Add that to probably not having encountered sexy roleplaying before, and I think he'd be unlikely to embark confidently upon it. I mean, I can see Ult Steve getting a clue and seducing Tony, with a view to later playing his weird Jedi game, but it strikes me as unlikely he'd just lightsaber up and grab him... okay, it could be made to work if you link it to Hallowe'en. After discovering Tony's fic, he asks Tony to sort him out a costume for the party. Tony promptly supplies a Sith costume, that he might take pictures and illustrate his epic. Steve considers that encouragement enough to make a 'Come to the dark side' joke, and when Tony's eyes glaze over, he deems it a plan likely to succeed, and gets with the seducing.

See, I think Steve would be very unlikely to aggressively seduce until he were totally confident of success; especially Tony, as a failed seduction would expose a massive vulnerability to a renowned piss-taker. REBUTTALS SHOULD TAKE THE FORM OF A FIC. :P