Yup, and running the Bolt in the most efficient manner possible when using the builtin heat. For example, recirc on and only tapping the defrost/defog button when fog appears just long enough to clear the condensation. Log heater wattage used by the Bolt for the entire run, and also monitor AC wattage in case the AC kicks on for whatever reason.
Based on how I understand how you're describing it:
The coolant heat exchanger and the combustion chamber are integrated with each other, then followed by a catalytic converter, then by exhaust with no further HX?
Yeah, putting an HX that preheats the coolant from the cat exhaust should increase efficiency, but you need to be careful that when starting to condense out the exhaust, you handle the rather acidic condensate properly. (This is a challenge with high-efficiency condensing boilers that are so efficient they condense the water vapor out of the exhaust.)
You might possibly find better efficiency improvements at that point for total system effiiciency than just making the combustor more efficiency - most notably as I've mentioned before by maximizing recirc by finding ways to control humidity without losing heat. As I've mentioned, either a dehumidifier or an HRV ( see
DIY Heat Recovery Ventilator (HRV) Plans for one example of an HRV - one to handle the Bolt efficiently would likely be much smaller. Take a look at Dan Achim's setup in the comments that was coincidentally posted only a day or two ago, I like his geometry that gets proper counterflow over a greater percentage of the unit, although I wonder if the tape he's using to seal the joints is going to create gaps that kill efficiency without a huge amount of thermal paste or similar material...