Yes, it can. I considered doing that but did not for several reasons:
1) Possible buzzing or vibrations from the rear panel from sub harmonics.
2) I use the rear shelf to pack stuff when on roadtrips and the trunk gets full.
3) Didn't want to drill into or mod the rear shelf in any way in order to secure the sub-woofer.
4) Footrest position is more in-line with car's low center of gravity versus it being higher on the rear shelf.
5) It would have been a very tight fit because of my car's harness bar.
Though, a lot of Lotus owners have their compact subs mounted on the rear shelf and are quite happy. A benefit is that it cannot be seen to someone peeking in. But then, most people looking at my car don't notice the sub in the footwell until I point it out.