Just a thought. How about doing a test mock-up using actual condoms. Get that to look right and then replicate that look on your sculpt? Nothing beats working with the original materials and use it as a sculpting guide.
For ease you can snap the lips right down the middle, so you have 4 separate pieces to mold. Molding it as a one-piece will probably be difficult to do as well as produce castings, but I'm not skilled with mold making, so maybe it's doable with a little guidance. You can easily make a sort of "snap-fit" system, so they always attach the same on the face and then all is needed is a little filler to close the seams. Just a thought. However, this will probably require the lips be sculpted with that break up in mind, which would mean you can use something like Aves Apoxie sculpt as the base for the 4 parts and have them connect in a certain way, so they hold together, and then simply sculpt on top of that thin layer with whatever sculpting material you like to work with best.