And exactly what stops you from defining each undirected edge twice, once in each direction, with the same maximum flow?
Oops, ta0kira posted before I wrote my reply. My thoughts exactly. You probably have to apply the constraint by canceling opposing flows on the same edge, but I cannot see any reason why it should not work reliably.
Last edited by Nominal Animal; 04-26-2012 at 10:40 PM.