Rpg Maker Developers Group

Title pretty much explains it, but here's a picture for visualization's sake:
And then imagine that as each character moves, the spotlight moves with them, like a stage play. So far, I've been able to make a single static spotlight by using a mask on the dim picture that looks like this (black spotlight on white background): The only thing I can think of is creating various versions of the mask depending on who is lit at that moment, and letting only a single spotlight follow one character at a time. Of course, this would all be much easier if there was a subtraction blend mode, but I understand that WebGL doesn't allow for it? Is there some other, more clever way of solving this problem?

Hopefully my explanation of the problem is clear. Thanks in advance for any help!