I've been using
NatNetDRM for a few years now and have been very happy.
I tried using one of the previously mentioned DRM solutions and it was nothing short of a pain in the ass to set up. In fact, the support between that particular company and one of the billing companies was so bad, it never did get set up properly.
Had NatNetDRM up and running in no time at all.
Also, Marc & Brandon have developed a DRM solution. Since you are in pretty well with them, you may want to hit them up about it.
