[FFmpeg-user] Blending two inputs with custom expression

[SviMik]

Let's assume A is a video, and B is a png image with alpha channel. I need
to do the following blending (assuming the format is rgba and B_alpha is in
0...1 range):

red = (A_red - B_red * B_alpha) / (1 - B_alpha);
green = (A_green - B_green * B_alpha) / (1 - B_alpha);
blue = (A_blue - B_blue * B_alpha) / (1 - B_alpha);

I was looking into "blend" filter, but it seems I can't access the other
channels data.
The "geq" filter is limited to a single source.

What shall I use it that case?
I really hate the idea of digging into ffmpeg sources and writing my own
filter. Is there any chance to do this kind of blending without that?