Thankfully I don't have to post another picture of a just-filled rubber mold! Despite my misgivings, yesterday's pour was successful.
The sprue from the chin did drain, but not all the way to the outside. I've tinted the filled area of the sprue blue, so it's easier to see where it stopped. Once again I got a concentration of bubbles on the underside of the jaw. That makes me think that redirecting the sprue to the left (yellow tinted line) might prevent that air from getting trapped.
Normally I would take this information and chalk it up to "I'll know better next time I cut some sprues". I have a good copy of the foal (the bubbles along the jaw are below the surface), which means I am ready to start making a working plaster mold. But I am going to be trying out two different versions of the Imp mold, and having two rubber originals will save me some time. Since I'm going to be pouring again anyway, I might as well try the improved sprue.
I'm also heading out tomorrow to judge at Clinky Classic, so it will be a few more days before I can work with the mold. With that in mind, I went ahead and ordered some fresh rubber to see if that might not help with the bubbles, too.