I'm not sure that approach would work.
Subtractive colors are used by inks mostly, where color is reproduced by the absorption of the ink of certain wavelengths. If you have cyan, then the ink is absorbing some red, and reflecting green and blue (that is only an aproximation of things, of course). when you have all three colors forming a dot, all colors from light will be absorbed, and you end up with a black dot.
Additive colors, on the other hand, are, well..., additive. You have green light, and any other color, for that matter. When you combine some colors, let's say green and blue, you end with cyan, which is the combination of both colors, not the absorption. If you combine all three primary colors, you know you end with white.
On the other hand, magenta is not a color you can find in the visible spectrum, as it's the combination of two colors that are at opposite sides of it (red and blue). So, you can see, if you have a magenta laser, you already have ona laser that can produce red and blue at the same time . Also, color rendition is not only a function of wavelength, but also on intensity (laser power, so to speak).
As for the combination of all three primary subtractive colors, in additive style, I'm not sure what the result would be, but black it won't be, for sure .
Does any one around here have a blue (argon or DPSS) and a yellow (HeNe or DPSS) laser that you can combine?, see what color is produced? :P
Remember the future?, That'd today, as you imagined it yesterday.