I received a reply awhile back. Due to the curve in the 30 round body they haven't been able to find a reliable way of pinning them to 5 rounds and still have good functionality.
I just had a try with my 5/20 6.8SPC mags, and they seem to feed the 7.62X39 quite well, but the 5/10 ones don't fair so well.
I've heard that the 6.5 AR mags might be the ticket for the X39 stuff though. Can anyone confirm? Case dimensions between the 6.5 Grendel and 7.62X39 are close but nowhere near exact, obviously.