I did try denim one time for a pouch when I was a kid but the problem is with the fray, it shot well with a light set of chains and light ammo heavy it could retrurn at times.
I do think that some stitching around the fray would make such a pouch more durable..
The guys in East and West Timor knit their own pouches from the cloth fibers of their rice sacks in the agricultural zones..
Here's two slingshots I won on an auction from West Timor showing a woven pouch, if this works I think a piece of doctored up denim could work.
I still think that you should always be careful with experimental materials.. Wear eye protection with this experiment..