A reverse proxy can be set for any protocol, not just HTTP/S. Having said that, for real-time protocols, like those used in a PBX system (e.g., SIP for signaling and RTP for media transmission), a standard reverse proxy might introduce latency. I believe there are specialized proxies that are designed for handling VoIP traffic . You could setup a specialized reverse proxy behind IPFIre (with port forward) that will take care of further forwarding the traffic to your PBX based on the FQDN.
Definitely the draw back of this approach is complexity. Maybe you could install banish; this plugin can use FQDN and could allow you to go around this IPTables limitation. It is not yet an official IPFire plugin but I believe it works quite well.