Ok,
On level 1 (burning pellets) with a feed of +4 and air of +1
At the top of the exhaust cover box on the cover itself I got a reading of 180 degrees.
In the middle of the box it was 160 degrees.
At the bottom of the round tube that goes into the exhaust box the reading was 245 degrees.

On level 2 - top 210, middle 170, top of tube 260 degrees.

It looks encouraging; i think you would be able to capture plenty of heat with a heat exchanger plate attached to the cover.
you may want to make a different cover thats a little heavier. but that shouldn't be that big of a deal.

Just for giggles i also checked right behind the burn chamber just below the fan
it was well over 280 degrees.

this little stove burns dam hot!

Measure the exhaust cover to see what dimension will fit. I think you should talk to people that have put them in and ask for their results. I know the smaller ones didn't produce much. The larger units worked much better to capture more heat. But you usually have to put the unit right above the flame for best results. Without getting into the arguments of free heat don't expect much from a idling stove regardless of the brand of stove. Simply put it takes X amount of fuel to produce X amount of heat. You can transfer this heat to other areas of your home using water but you might be running the stove at a higher heat level to do it ending up that the room the stove is in will be to hot. Each home is different so what works for one won't always work for another.

Cool the exhaust to far and you will be replacing vent pipes soon.

How about running some plumbing inside a few of the exchanger tubes?