I managed to create some PHP scripts that parse Icecast XML statistics into an SNMP response, so I can add them as a custom OID to the SNMP daemon which run on the streaming machine. Then I can query that OID with Cacti, see:
I think that my simple PHP scripts can be easly converted so that they integrate directly into Airtime to give a real time reading of connected listeners. To have to reimplement all the RRD stuff, that would be a different story...