Microsoft, as far as I can tell, does not support Additive and Subtractive blend modes in Avalon. One performance hungry way to do this (Additive only) is to make your object 3D and then apply a EMMISSIVE shader to it. Works better if its a bitmap.
Nathan Dunlap told me he was able to simulate bled modes with the help of DirectX Filters. I think if it can be made into a simple reusable model, it would be perfect. Will experiment on it after February 10th.