Fly plaid with 4 yard box pleated kilt??

[MacMillan of Rathdown]

I'd generally suggest "no", regardless of the pleating of the kilt. But then I'd also voice a "no" to belted plaid, a piper's plaid, or a lairds plaid. In my experience plaids just get in the way at formal events whether dining or dancing and tend to look a bit O.T.T.-- the exception being the groom at a wedding.

As Rocky has pointed out, it's not the pleats that make a kilt formal, but rather the style of jacket and accessories.