WARNING: When it comes to automotive, I am dangerous, so once you have finished laughing and picked yourself off the floor, please set me straight.
I note that that the Orion can take a wide input voltage of 8-35V.
I also note that it is intended to be on the far side of the starter battery and charge using of the alternator current that the starter battery is also connected to: 
Consider this use case: An over-landing 4x4 with 200W of solar panels and two lead acid batteries in parallel (with an isolator).
What would stop us from putting the output from the solar panels (unregulated) into the Orion as well as the alternator and then charging both 12V batteries in parallel from the Orion's output.
The isolator would need to work as follows: - Both batteries would need to charge while engine is running - When not running, the starter battery would take priority for charging but once it reached float, the Orion should switch to the auxiliary battery to charge.
If it was unwise or otherwise unworkable to use unregulated solar, then what about adding a MPPT 75/15 between the panels and the Orion's input?
