I would go with the oversize bearings and make them crescent or U-shaped like most of the new wave of atm scopes. You don't want to carry around an extra 7 kg for no reason. The only disadvantage is that they would not have a point of rotation to mount encoders on but that can be solved too. Also need to make sure that they do not get too long, thin and springy. Many people make them removable but for a 12" it's probably not necessary. Some examples:
http://www.fiskemiles.com/buildinga2...e-finished.php
http://www.biophysik.uni-freiburg.de...owrider_e.html
http://www.stathis-firstlight.de/atm/kyklopaseng.htm
http://freenet-homepage.de/atmpw/Bil...l/default.html
A spring type "virtual counterweight" has also been used successfully.
Good Luck