With a combination of IPSC_Bridge, Analog_Bridge,  Allstar Asterisk, and an AMBE chip you can connect a SIP client to a single DMR talk group, however changing talkgroup on the fly would require some additional code. 

 

You'd also be limited to one talkgroup per AMBE chip. However several SIP  users could connect to the same TG. 

 

I *intend* to make a Web based client for Allstar Asterisk, which could be ported over as an app with some assistance, but it's a little way down my list of projects. 

 

I hope that helps

 

Matthew