pfSense

aside from the difficulties Jim noted, it's also a really bad practice to rely on a single IP (even anycasted) for determining the availability of > 1 WAN. You don't want to do that.

Actually, I DO want to do that. Also, the same destination but different origin are distinct from each other. If there is a technical limitation that can't handle that, either it needs to be worked-around, or it needs to be fixed eventually. I can dynamically ping via any interface I want without needing OS-level static routes, why can't apinger do that too? Some easy examples:

ping -S 192.168.0.2 8.8.8.8
ping -S 192.168.1.2 8.8.8.8

I'm guessing there might be other reasons why you need a complex solution for a simple problem (pinging an IP), but maybe at some point the issues blocking this can be simplified so they can ping any arbitrary monitor IP. If necessary, maybe a quick and dirty workaround would be to have the gateways take turns pinging their monitor IP's, so only 1 route is in operation at a time.

the problem is you must hit the IP via multiple WANs simultaneously to allow that. As things currently stand, that's impossible. Multi-FIB support at some point in the future might allow this. Until that point, it's not possible (and something like changing routes around would be insane and no doubt buggy).