I finally decided to acquire an AIS transceiver for Blue Opal, and went with one that talks both NMEA 0183 and NMEA 2000; an Alltek Marine B600s. It should happily chat with the Miniplex3, which will deal with sending the received AIS transmissions to something like my iPad (as there is no chartplotter or MFD on Blue Opal yet). It arrived from Germany (SVB) in less than a week, and configuring it took all of 30 seconds.
Double, triple and quadruple checked the MMSI 
The patch antenna getting a 3D fix 
Confirming that the device is seeing sentences (internal)
Alltek’s configuration software is decent, though I think it probably makes sense to do a “have you really checked that MMSI?” warning on the first save.
The unit wasn’t getting any GPS signal indoors, but once I chucked the patch antenna through the window, it found the first three satellites in under 5 seconds, and had a 3D fix in about 30 seconds; modern GPS chipsets are amazing.
The log confirmed that various sentences are showing up, so all I need now is a SD card to act as an AIS data log (for now, until my data logger project comes to fruition), and a space to install the unit in Blue Opal.
On the bench power supply, with 12.6V on the dial, the AIS is pulling 0.25A steady state.