I wouldn't be able to test whether this works or not as Aero is disabled on my system, but you may be able to do this with this program: TransBar
Run it, set the "Taskbar Transparency" slider all the way to the right, or maybe even at 99%, and see if it works. I don't know if this program adds to pre-existing transparency or overwrites it, but it's worth a shot.
My Computer
At a glance
Windows 8 Pro 64-bitIntel Core i7 950 @4.4GHz12GB DDR3 1333MHz @1400 (9-9-9-24)2x AMD Radeon HD 6770