Clipping the autoblock to my leg... yes. Since it's supporting almost no weight... no.
Your autoblock should be attached to a load-bearing connection point. You'll find some rock climbing harnesses have dedicated loops near the thigh while others don't. In the case your RCH doesn't have a dedicated leg loop, you would extent your rappel runner (going to your ATC) and clip your autoblock to the belay loop as well (below your runner).
Now with the technically correct description for safe use is out of the way... the Aero Evo (my saddle) was made with leg loops that are load bearing and I attach my autoblock to one of those. If your saddle isn't an Evo (I feel sad for you
), your lineman's loop *MAY* be an acceptable place to locate your autoblock. I would check with your manufacturer for their recommendation before attempting to use them this way.