Driving expressions with pixel data from a specific point?

Does anyone know if there is a way to have a tool that samples a pixel at a point and then publish the R, G, B or A data from that pixel? For example if I wanted the alpha of a pixel to drive the blend of a merge somewhere in the flow... this would be insanely helpful, but I can't see any way to do this with the default tools

Right click on any parameter and modify with Probe.