I have to concur with cajunscot. I have both modern (Utilikilt) and traditional kilts, but I don't try to mix looks or accessories betwixt the two. A clan crest seems kind of out-of-place being worn with a modern non-Scot, non-clan kilt. But that's just my opinion, and it's worth exactly what you paid for it!

Practically speaking, yeah, the apron on a UK is a bad place to put a pin. It's too far to the inside, and will end up doing a lot of banging around as you walk.

If I may ask, what "special occasions" are you referring to that you're planning to bring forrit the clan crest for?