Keyframing a layer selection dropdown

Set Matte, Calculations, Displacement Map - what do these three effects have in common? They all rely on another layer to drive them. There are many other places in After Effects where effects need to refer to other layers and it helps AE be a powerful tool.

However, there's a big limitation.

There is no ability to keyframe this parameter. When referencing another layer, sometimes it'd be lovely to switch what layer it is! I know it'd be a hold keyframe and that's fine. But this seems like a really useful feature to have.