multiple ISP + multiple FIB, add routes from DHCP to different FIBs


We just got connected to an additional ISP for redundancy. I'm already using multiple FIBs for selective routing for specific subnets.

Problem is: both ISPs assign IPs via DHCP. The IPs are static, but if no DHCP-lease is aquired the connection is dropped after a timeout period.
So I need to configure both interfaces for DHCP, but the default gateways should be assigned to different FIBs.

I tried in /etc/rc.conf:
ifconfig_em0="setfib 4 SYNCDHCP"
ifconfig_em1="setfib 5 SYNCDHCP"
but the default routes still go to FIB1.

I couldn't find a configuration syntax for this scenario in rc.conf(5), setfib(1), setfib(2) or dhclient(8)...

I know about /etc/dhclient.conf, but IMHO it should be obvious from the interface configuration in rc.conf (as a "single point of truth") that the routes go to other routing tables and I'm sure I only get the syntax for it wrong ;)