The only way to bridge USRP across more then 2 connections is with ASL.
The other issue to consider is that you don't ALWAYS want to go analog for conversion. Consider this:
DMR, NXDN, YSFn.  No conversion required. All 3 are AMBE so it's a simple cross connected MMDVM single instance for 2 modes.
YSFw and P25. No conversion required. All 3 ar IMBE so it's a simple cross connected MMDVM single instance for 2 modes.
D-Star is stand alone.

So, for example if you used 2 instances of AB to bridge DMR to NXDN you would loose quality by going to analog when it's not needed.

 Steve N4IRS