You could try:We have tried it this way, of course, and it works. But the corresponding routes are static. In our context, all routes have to be dynamic (for fault tolerance and due to configuration overhead). Zebra and OSPFD do not support this type of route. As a consequence, this approach does not work for us. Anyway, iptables is the way to do it.
ip route change default via <gateway> src <vipa>
Presuming the NFS clients are behind a gateway. If also onlink, you need to modify the connected routes and change the src there too.