I believe most services allow you to wear any insignia you earned (rank, qualification badges, unit patches, etc) in such a manner that does not bring discredit upon the military.

I've seen plenty of USMC Eagle, Globe and Anchor insignia worn as a kilt pin, bonnet badge (have two myself), on a belt buckle, or on a sporran.

Personally, I think wearing your rank as part of a kilt pin would be entirely appropriate.

Sir William
USMCR, Ret.