In general you should wear what you like. OTOH a gentlemen should knwo the rules first so he can break them intentionally rather than willy-nilly.

I think the consensus will be that an American might wear a fly plaid as a groom in his own wedding, and then not again until inducted to the House of Lords by Her Majesty, or promoted to colonel in a UK regiment. There are a few rare exceptions, pipers and the military have their own dress codes of course; the introduction of a new sett likewise might be an opportunity for a but more flair than ordinarily observed.

The underlying question is are you wearing the kilt as costume (which would allow one to wear a fly plaid with great regularity), or as proper respectable men's wear.

I for one would not say anything denigrating to someone at a highland games in full on highland dress complete with plaid, but I would be leaning on my situational awareness skills to ascertain if the individual was a visiting clan chief who is representing, or not.

I have no idea about the actual Smithsonian gala, but even if I were planning to go kilted I would almost definitely not wear a fly plaid. Perhaps an email to the host(ess) in advance? IIRC a fly plaid is an optional part of white tie dress not generally associated with "mere" black tie.

But at the end of the day, you are an American and should wear what you like.