I created a patch which enables editing the alpha channel of vertex colors in the vertex paint mode. For this I had to extend the MeshColor and MeshLoopColor to four channels to store alpha values. Then I introduced another blend mode (set alpha) to the vertex paint editor. I used Brush.alpha (which is also used for intensity and pen pressure I guess) for setting the alpha values instead of extending Brush.rgb to Brush.rgba which I tried first but noticed it was too much trouble.
I implemented OpenGL rendering for solid and textured shading for immediate and VBO mode. But because of transparency I still need to sort the triangles depending on the view. Does functionality already exists which I could use for that?