His very first step would be to tell Nonnally. She was nervous about coming out to big brother Lelouch and he wanted so badly to tell her that she didn't need to be worried BECAUSE HE IS ALSO GAY but he couldn't. Then he would tell Nunnally. And he would swear both of them to secrecy, because he wants to be sensitive to Euphie's feelings, but I think he could be talked into telling her on the basis that it would hurt her more if she found out from someone else.
Other people he would "come out to" -- Yuuri. His sex ed class, but more in an abstract way. (No naming names; it's just for the education on sexuality part.) All of his other close friends.
Really, if it weren't for not wanting to hurt Euphie, Suzaku would string up banners everywhere. And in that situation it's not about not wanting her to EVER know -- it's about trying not to make a big show out of how... happy he is with Lelouch, you know?
I think I've convinced Lelouch to never lift the prohibition ever.
Some outside alien force, like GIR but less easily fooled, demands everyone in the dome have at least a threesome. Suzaku is okay with it, but the choice is completely up to Lelouch; who does he let into their bed for one night and one night only?
(Also popped into my brain as "forced to have babies; Suzaku wants to do it as a threesome for minimum cheating". But C.C. is too obviously the choice there.)