Can't vcol_light be set as emitter?

Been trying to set up a better PRT baking rig, so I made a sphere and raybaked it from red, green and blue emitting disks. The idea was to use this baked sphere as the emitter to bake actual models.
That first step worked, but when I try to use my sphere as an emitter, I have a problem. As you can see in the pic below, I set up the material as vcol_light, and made it to emit; but when I try to raybake my gun turret from it, it seems that the diffuse color of the sphere is used, instead. If I set it to white, I get normal radiosity baking --white light; and if I set it to black, the radiosity terminates right away, as it sees no emitters.

Isn’t there a way to ge vertex color info to emit? I tried vcol_paint with the same result…
(Ehmm… I’m using the Halloween cvs Release, if that should make any difference.)