This sounds like a fascinating challenge. I think I'd like to help out. My first suggestion would be to cast both front legs. That sounds much easier than it will be I'm sure. You can search Google and find out what people are using to cast body parts. Alginate, a dental casting rubber might be the best. Then you'd do a plaster cast from that mold. From that mold you'd start designing and building a front paw. You'd need to decide if you want any articulating parts, like a foot on a little bit of a hinge to facilitate walking.

The first step it to cast the legs and then figure how to hold (whatever the printed part is it) on. The second step is to test it, modify it, test it, modify it and sooner or later, have a design you could use.

http://www.dickblick.com/products/ar...pbWRoCDczw_wcB

So, pull a balloon or condom over those legs (that will smash the hair down and simulate the dimensions the prosthetic will need to grip to) and cast them. Maybe a doggie "downer" will help with the process. I don't advocate drugging animals but, if it helps you walk someday it might be worth the one-time buzz. Althought, that looks like a very sweet dog. I bet it would let you do anything you wanted to it and not even fuss.