I've got about ten leather kilts, I never bothered putting a kilt pin on any of them. I think 21st century kilts makes a kilt pin available for their brand.
There are a couple of ways to do it one could sew in a piece of metal in the hem fold over at the appropriate spot so a magnetic pin could fasten. If it were permanent punch two fine holes for the traditional pin to slide through and lock.
Cheers
Robert
The leather and hemp Kilt Guy in Stratford, Ontario