Is there a way to transmit a CAN bus signal over a high current DC line?

You can build a simple filter network to block DC.

Here's some design examples:

Electronics Stack Exchange - DC Filters

That URL came from the Electronics Stack Exchange, which might be a better place to ask the question.

In theory it is possible. It would require 2 series capacitors to couple and decouple the CAN signal onto and from the DC line. After the first capacitor you would have a diode to add the DC voltage to the CAN signal. Between the 2 points in the circuit the CAN signal + the DC are combined. At the second point the second series capacitor would pass the CAN signal but block the DC. The diode before the second capacitor would recover the DC voltage. Then the recovered DC would have to be filtered to remove any residual noise. In Theory... The values of capacitors would probably be small like .001uF disc ceramic. I believe this process is referred to as phantom power. I hope this makes a little bit of sense or a least teases you a bit to do a bit of extra research.