Connecting devices powered by separate SMPS without common ground
After looking at several options, I settled on using the SMPS in the SI3402 for the 3.3v rail as it will be constantly on and drawing 250 - 300mA (mostly for Ethernet), and boosting the 3.3v to either 5v or 12v using a LT1377 switcher which is a cheap but efficient switcher. The LT1377 can be enabled/disabled by the pic which will save heat/power as the sensor only needs power for occasional reading, and the LT1377 is 85% efficient so not much extra power is lost.