Hello Brother,

I've known some to wear a fly plaid with their mess kit, so it can work I think for a PC. Personally I think it doesn't look as sharp, but that's just me. I've never donned a fly plaid with my mess kit for instance.

Since it's your granddaughters wedding I think you can ignore most "rules" and just wear what she wants, especially if it makes her happier on her day

As for using a "Munitions Maintenance or EOD badge as a brooch" you don't have to. You could attach it with a standard brooch of some sort and than add that badge either to your PC lapel, or if it's too big to go there, attach it to the fly plaid just under the fastening brooch. It's not quite standard, but it still works. I've seen some people place their metal jump wings there until they got proper lapel miniatures.

Slainte!