Update:
I have good news and bad news. The good news is, the silicone I've selected is excellent and will be perfect for casting the vents out of. The bad news is, the driver side vent body I used as a trial mold may be unusable. The master came out completely shattered, due to some oversights I made in the mold design. I don't even know if I'm gonna be able to salvage this master. If all this is true, the project has just gone way over budget, and way past deadline. I'm still waiting on my source for the vent body I didn't have, and now I may have to source another vent from an ever-scarcer supply.
I'm going to do a trial pull tomorrow. I'm not holding my breath, but it is possible the initial demold was just more than 30 year old plastic could handle, and fresh urethane in a broken-in mold may hold up better.
Keep your fingers crossed, Club S12.