Why not emphasise why borrowing a personal kilt is not 'the done thing' or a good idea.... with the reason being that you wear your kilts in the traditional manner... in the full sense of the word... (wicked grin) after all they wouldnt want to borrow your boxers, y-front, briefs or xxxxxx (substitute with suitable underwear)!

You could suggest they get a travel rug and a belt and show them a site or two where they can learn how to make a great kilt? They can go to their party all 'Rob Roy and Braveheart' and less 'Sir Walter Scottish'

The only time I would lend a kilt is to family for some special reason...