Making projector-based selection circles work in air

I'm currently using projectors to draw selection circles around my RTS units, and I have a very nice custom shader facilitating this. However, the problem is that I want the selection circles of flying units to appear in the air so that you can clearly see which ones are selected. My first thought was to add a transparent plane underneath the projector, but I'm guessing, that projectors won't project onto things that are transparent because they have no depth assigned in the Z-buffer.

What i want is basically for the circle to be projected onto the ground if the object is near the ground, and at a plane if it is in the air. Are there any good solutions for this?