How to add a custom blend mode?

[Reekin245079453yt6]

It seems there are no many discussions about this. I've read some related posts but haven't got a clear answer.

Basically I want to define a formula to decide the pixels' displaying color between 2 layers, such as "r = r1 + r2 * 0.5f",  jusk like writing a shader.

Since I need realtime feedback when using the new mode, so I can't accept going arround solution that makes the process complicated. It should works just like a blend mode.

Is it supported?