I've never used XCP-NG but the keyword to look for, that most Linux distributions apply for this kind of route outside of the interface's subnet, is "onlink" (pretend that the nexthop is directly attached to this link, even if it does not match any interface prefix).
A workaround could be adding a SystemD service that runs before XCP-NG's networking service and whose only job is to run "ip route add 100.64.0.1 dev xenbr0"? I don't know if that's possible, depending on whether xenbr0 has already been created by then.
That’s a solid observation. Since XCP-NG doesn’t natively support routes outside the subnet, filing a bug report makes sense. In the meantime, using a systemd service as a workaround seems like a practical approach, provided the bridge is up in time.
It actually reminds me of the Bowmaster action game: sometimes the default mechanics don’t cut it, and you need a creative workaround to hit your target. Same idea here with networking fixes—different path, same goal.