I'm not sold that a cooler (fans) will cause condensation.
100% correct. The best a fan can ever do is to get an object to the temperature of the air it is blowing, i.e. room temperature. Condensation is caused by the air being too cold for the amount of moisture it contains. So unless you are using your MBP in a sauna, or some other environment where water is spontaneously condensing on the walls, your desk, and yourself, no amount of fan blowing is going to cause condensation. (In fact, if anything, fans make condensation less likely as they will warm the air going through them slightly.) You'd need real active cooling, like a refrigeration unit, to have that happen. Or, I guess having large amounts of extra moisture added to the air, although I can't think of any sources which would cause that, which wouldn't damage your computer even without using fans (like a sauna).
EDIT: One caveat to that is a fan in a pull-configuration would work like a refrigeration unit, and has the potential to cause condensation. However, I doubt any cooling pads would use this, and even if the did, the chances of it happening to a noticeable extent I'm sure would be minimal. /EDIT
My first observation on the original point would be that almost all the heat from the CPU is going straight into the air via the radiators, which is how the cooling system is designed. Only a small amount of heat goes through the bottom of the case, and it moves slowly, evidenced by the time it takes to heat up the case, and the time it takes to cool down afterward. This small amount of heat is unlikely to be noticeable compared to the heat doing what it is supposed to, and being blown out by the internal fans.
Instead of investing in a fan powered cooling pad, I'd suggest doing as others have said, and raising the computer a little above the desk, especially the back. That way if the base does get extremely hot, it will generate it's own air current to cool down. This would work best if air can get in under the front.
My other suggestion would be to ensure that the hot air being blown out from the fans cannot be recirculated back to the intake vents. Probably the best way to make sure this won't happen is to have a fan somewhere in the room, which stops air stagnating.
-End of wall of badly explained science.