Send option/alt key press to OS X connected from Windows via VNC?
Solution 1:
Problem existed as of synergy 1.3.4; was resolved after upgrading client (running on the Mac-side) to 1.3.6p2.
Solution 2:
I use Vine VNC server on Mac OS X Leopard, and RealVNC as the client.
My Ctrl key works as Ctrl, Alt key works as Cmd and the Windows Logo key works as Option/Alt. I guess thats what you want, so perhaps try running Vine VNC server on the Mac.