[Serusers] SER and multiple URIs for UA

Hi, I'm trying to integrate small ISDN networks into a SIP network, so quite the opposite of the usual way (SIP -> PSTN). There exist some SOHO ISDN-to-SIP Gateways with two BRI out there, and I've successfully tested some of them by using the user id of the gateway as MSN on the phone. Now this is quite a waste of ISDN, cause you can in fact address up to eight devices on an S0 Bus. My current solution is to use the alias table of SER to configure the MSNs for the Gateway. The Gateway itself registers with e.a. sip:011000@my.domain and there are aliases from 0110001 to 0110008 pointing to that URI. But this still does not satisfy me because I think the usual way would be to configure one base number which the ISDN2SIP-Gateway uses to register with my SER, and let the user assign as much phone extensions as he likes without any interaction of mine. Here I'm thinking about a loop in the ser config where I lookup the location of the user part of the To-Header (if "^[0-9]+$"), and if not found, just cut off the last digit and repeat the loop until the user id under-runs a minimum length. If no appropriate user is found, I return 404. Otherwise I forward the original URI to the IP of the user I've found. So, generally said, I assign base numbers and perform some kind of wildcard matching on incoming URIs. What do you think about this solution? Would this be appropriate, or do there exist other approaches? I'm looking forward for some comments, Andy