It'd be extremely unlikely. Platies and Mollies are in different genus' (Xiphophorus and Poecilia respectively), whereas horses and donkeys are both in the genus Equus. It'd be more like a dog trying to breed with a fox; they're both canids but different genus', and viable young would be practically impossible.
I've heard reports too but I highly doubt it. A quick search on the net comes up with no images or definite proof so I'm sticking with the opinion of no, it's not possible.
It is however possible for swordtails/platies to form a hybrid, as they're both in the Xiphophorus genus, and to a lesser extent Mollies and Guppies, being both in the Poecilia genus. I don't, however, suggest it as a good idea, more so in the latter case.