Had a guy at the range tell me that pinning a retractable buttstock in place so that it is rigid on a post-ban AR is legal. Is this true? I'm sure overall length is a concern, but I have a 20" h-bar so it shouldn't be a problem in that department. If so, how is it accomplished? I would like to put a VLTOR Clubfoot on my AR as I really like the aesthetics of them. If it isn't legal please let me know as I don't want to waste my money thanks.

It is perfectly legal as long as the pinning is permanent, such as welded pins, or just welded.  The point being that however you do it it should not be able to be easily converted back to a collaspible. You can buy them this way as well.

Moreover, as you pointed out the total lengh of the gun must meet the legal requirements (26" as best as I can recall.)