I've run a couple of stripper clips through one with the stock on, but not with the stock plus sling. It was cool. The manual of arms with loading the stripper clips just isn't as smooth as popping a new mag into a modern pistol design.
Short video about the C96 with buttstock. Really nice pistol, but looks a bit difficult to reload, though. Is it just the guy in the video or is the use of stripper clips really a bit tough?
You really have to put everything in the right context. Mauser C96 dates back 120 years or so (pre-dating the 1911 morethan a decade), of course it is not have the finesse of a modern service pistol. This gun was the first real alternative to service revolvers, and I guess it could be said that it was the proof of concept and first magzine pistol capable of frontline use. I believe even Winston Churchill himself used one in his early career when he was fighting in Africa.