Add/remove route on tunnel up/down
is it possible to get pfSense to add/remove a route when a tunnel goes up/down?
no, as there are no routes with IPsec (assuming tunnel mode). Transport mode plus a tunnel plus a routing protocol would be the answer there.
I was hoping I could use BIRD and get it to advertise the relevant VPN route into the network when the tunnel came up. I haven't worked out whether that is possible or not yet.
You could hack in BIRD if you wanted, but Quagga is already in packages for OSPF and would be a better option since it's integrated. That's an option with IPsec transport mode and GRE or gif, or OpenVPN.