Squig, I was on propecia for 10 years, from 20-30 years old. I was diagnosed with MPB at 20, and propecia worked wonders. Serious hair regrowth man. I started lifting when I was 22. I weighed 135 lbs. I lifted naturally for 7 years, gained 40 lbs of muscle over that period, and never had a single issue from propecia. I have tried to do an enormous amount of research on this exact issue (Propecia and muscle growth/inhibition), and have found very little. As I'm sure you've found, there isn't much out there on 5 alpha reductase inhibitors and exercise/muscle growth. Most of the science talks about sexual dysfunction, and I believe this affects about 1% of propecia users. There is actually a brand new study (last week or so that came out on this) indicating that the sexual dysfunction from propecia can be permanent, but is very variable between individuals
All I can say is for me, I never noticed an inhibition of gains, or any reduction in any androgenous characteristics.
Unfortunately, I stopped using propecia a little over a year ago, and the acceleration of hair loss is quite quick when you do so. I didn't start losing hair at the same rate I was at 20-21 years old, I started losing it fast. After about 6 months, I tried to go back on propecia, stayed on for 3 months, and unlike my original go around, I had no cessation of loss, and no re-growth. I have no idea why it was ineffective this past time (desensitized pathway maybe?). Was propecia worth the money? Yes. Do I think it inhibits muscle growth. No. I suspect that it would be even less likely to do so in the presence of AAS. Hope this helps a little.
GS