I...think that a kilt and a nice tailcoat would be accepted at all the balls except the most anal-retentively period ones. For what it's worth, I'd guess at least a third of the people in attendance were not actually period, just formal.

I'll go one further. It is my memory, although I can't pull sources at the moment, that previous EAS promotional materials explicitly stated that kilt + Prince Charlie was acceptable attire.

I believe further discussion stated that the attire was period [newly introduced...note the Charlie there, though I'm again not with sources at the moment so take this all with a grain of salt], and that there's documentation of Scots wearing the new national costume to formal balls in England.

So, yeah. I'd double-check before taking my word for it, but I've been told it would be accepted and believe it would even be fairly correct.