An addendum to my previous post. This setup seems to create an Asymmetric Routing issue. I fixed this by creating manual firewall rules per https://doc.pfsense.org/index.php/Asymmetric_Routing_and_Firewall_Rules.
In addition, outgoing packets appear to be NAT'ed (they all appear from the outside come from the gateway address). I've experimented exhaustively with the NAT settings and have not been able to resolve this yet. I'll post an update if I figure it out. For now, it hasn't affected any of my services, except that e-mails appear to come from the gateway address, not the mail server address–necessitating some PTR record tweaks.