Of course I figured out the answer myself once I started digging around a bit more. I'll leave this here in case anyone else comes looking for something similar.
The solution…
In OpenVPN custom options, add...```
--route-up "/sbin/ifconfig bridge0 span ovpnc1"
Bear in mind, I'm using this to carry the output of a span switchport over to another network in another location, hence 'span' in the command above. If you just need to join the bridge, use 'addm' instead of 'span'.