Can a lower cabinet be used over a floor vent?
Yes, it can be done.

Relocating the vent, however, is a better general solution because:


The HVAC system can operate closer to its design. Reduced size registers and additional bends change the back-pressure and can readily throw the system out of balance.
Specialized registers and other parts are not required. 
The less common installation is more prone to errors than typical industry practices.


None of which is to say that this is or is not the right solution for any particular installation, but rather than it is really only the preferred solution for a set of edge cases.