I documented what could be achieved here, however, this will require some work on the MAAS side: https://bugs.launchpad.net/MAAS/+bug/1737428
Using multiple gateways requires either being very specific in terms of static routes, policy rules or having dynamic routing. Using kernel VRFs for sending requires application or automation changes to make sure sockets used for sending are bound to interfaces enslaved to VRF interfaces.
Meanwhile, for responding via the same path the traffic have been received policy rules that do not depend on remote subnets work quite well (cidr below is a subnet configured on a node, not a CIDR a client is connecting from):
https://git.launchpad.net/~canonical-bootstack/charm-policy-routing/tree/templates/service.j2
ExecStartPre=-/sbin/ip route add default via {{ gateway }} table {{ table }}
ExecStart=/sbin/ip rule add from {{ cidr }} table {{ table }} priority {{ priority }}
ExecStart=/sbin/ip rule add from {{ cidr }} to {{ cidr }} lookup main
The only case where is does not work is unbound udp sockets. It does work with bound and unbound TCP sockets and bound UDP sockets (DNS servers such as bind9 bind UDP sockets to all addresses to avoid this problem specifically).
What I found is that for many cases where sending traffic is needed one default gateway is enough in addition to a small amount of static routes.
The example given above might appear as a feature in MAAS soon.