Hi,

I believe that such balancers are possible. I have seen
such a system on a washing machine probably
made in the 1960s. Huge initial shake, big clunks,
perfect balance.

http://www.monografias.com/trabajos1...ensating.shtml
Very mathemetical paper but may be a start for a search.

A while back I did a bit of googling on such balancers but
I forget the results now but seem to recall an application
in some reasonably high volume device. Chain saw?