I have a friend who modified an Eton jacket; he didn't add any silver buttons, but black covered ones, and wears it when he doesn't want to wear his PC jacket for black tie functions. It is very similar, as others have already mentioned, to a "mess dress" jacket.

Fly plaids: I wore one with my old Modern Gunn kilt at my wedding in 2002, and to a couple of Burns Suppers, but really, they're more trouble than their worth. You can get them caught in a myraid of things -- chairs being the number one -- and when I ordered my Cumming kilt, I didn't bother to order a matching plaid, just because I like to keep the PC simple.

T.