How/where is the NMEA 2K decoding being done? OpenCPN able to decode 2K? 2K first converted to 0183 by? etc. Thanks.
Consider using the Beaglebone Black. Its got an on-board ADC. Generally better equipped for interfacing to real world versus the Pi.I've built some simple things out of Arduino. Looks like from a brief search the Raspberry Pi system doesn't have an on board A/D converter, but people have built them and there maybe some kits.
If you have analog sensors for things like oil pressure, I don't see why you couldn't design a high impedance interface that would just go in parallel across the sensor with whatever gauge you got to measure a voltage level without impacting the gauge. Then you could infer a resistance level and translate in code to an oil pressure for example.
I've never really dug into the NEMA stuff, so no help there.
Warning, every time I start messing with some hack like this, it's more work than originally planned Best of luck.