Kev, so you mean a greyscale image, the darker it is the less specularity, full black being none, and the lighter it is the more, full white being crazy-reflective?
Hrm... I don't know. That might be interesting, but I think the end result would look "painted on" instead of the real-time way it handles it now.
If the alpha layer were pure on/off (full black and full white, no shades in between) that might be more useful...
Then again I don't know for sure, I haven't thought about it much.