It seems you are right, it had appeared that there were screw holes beneath the rubber feat but having just prised one off it's not a screw hole after all, just a 1mm deep notch. Still it might be drillable, I guess I'll find out.
I'd thrown a Pibow Coupe case in with my order anyway, though it wasn't the one I wanted to mount, but for a second RPi which I use for prototyping.