I currently use the opacity channel of particle color data sent to the GPU to store other information. Adding another channel of float data would have a detrimental performance impact when millions of particles are visible. I'm not sure I really see the benefit of semi-transparent points in the view that simply assigning a semi-transparent material to regular particle geometry wouldn't already solve...
Another workaround would be to set particles to sprite mode and then assign a semi-transparent solid color texmap to the "bitmap texture" sprite parameter. I know that's a lot more difficult than simply changing the value of the color picker, but it will accomplish what you need to do.
Another workaround would be to set particles to sprite mode and then assign a semi-transparent solid color texmap to the "bitmap texture" sprite parameter. I know that's a lot more difficult than simply changing the value of the color picker, but it will accomplish what you need to do.