I did a little more research. Called Navionics and Raymarine.
Navionics says that their iPad app is primarily intended as a stand-alone app and told me to contact Ray regarding interfacing with a chartplotter. That seemed odd considering that Navionics developed Plotter Sync and incorporated it into their app less than a year ago as an interface to Raymarine plotters.
Ray says that they have “stepped away from Plotter Sync” because it was not reliable. I suspect that another reason might be that it competes with their new E7 chartplotter’s WiFi connect features.
My takeaway is that the Navionics app is probably useful as a nav tool and it MIGHT interface, but since I’m looking for an instrument repeater I’ll be looking at the iNavX app instead. iNavX is a bit more expensive, but it gets good reviews from folks who are using it as a plotter and real-time repeater