The motivation (and back-end needed) for doing your own thing with SIP (SIP2SIM) and SMS (delivery by HTTP) are quite different - often you want one without the other.
Currently, you can take control over SMS while using AAISP direct routing for the calls, but not vice-versa: once you select SIP2SIM, you are forced to handle the SMS yourself. In fact, the default setup gives you a flaky arrangement where outbound SMS from the phone is delivered with a null originator, and there's no way to reply.
It's almost possible with the current configuration scheme, by entering a suitable URL in the 'MO SMS URL' box, something like:
but this doesn't work (presumably due to the extra 'iccid' parameter that the outbound sms.cgi doesn't want).