Is it a register, or a cold air return? (ie, does air come out, or get drawn in?)

Maybe the item I've linked to below would help. You're right, it's not ideal to do what you're proposing, but this may work for you.

It is a register, air blows out of it.

I couldn't see the link Alan, did it come through? Thank you!

maybe a diverter would work.
something like this:
http://deflecto.com/products/pc/Air-Deflectors-Vent-Covers-c18.htm

otherwise don't put furniture in front
of supply..or return.

best of luck.

Whoops, my bad. Energy rater linked to the exact same site.

I've seen the odd diverter in places like Home Depot, but that site has a huge range....