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Yeah, sorry if it wasn't clear in all my previous info, but the LCD brd is intended to drive an analog gauge directly and assumes a ground referenced sender.
Analog Devices makes digital pots that work up to +30V. The AD7376 comes to mind.
The problem is going to be finding a digital pot with such a low resistance. 10K~100K is the normal range of these devices. You can't really parallel something like this with a 200~300ohm resistor and expect to get somethig useful. It will be terribly nonlinear with the pot only impacting overall resistance near the end of its overall step range. Maxim makes a DS3906 for low ohm applications, but its 5V, so stuck again.
I suppose you could build your own low resistance ladder network with several analog switches (silicon not mechanical) and resistors. Need at least 6 bits (64 steps), but now you need a 6-bit parallel interface to the micro. I'll think on this some more........
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