Hi David, there are only one or two owners of your furnace on the forum.. Personally i'd contact the manufacturer.
I will offer some insight,, most hot water coils http://www.hilkoil.com
are meant to be installed in the firebox itself.. the coil needs the intense heat inside the firebox to raise the water temp enough to be effective.. if a coil heat exchanger is mounted in the hot air plenum it would have to be quite large, with a lot of surface area to extract enough heat from this much cooler area in the furnace..
I don't know how wide the air gap is between the firebox and the outer wall of the hot air jacket.. but you MUST be able to get to BOTH sides of the firebox wall to seal the coil to the firebox.. otherwise you will have the potential for a direct leak of Carbon Monoxide directly into your hot air plenum and into your house...
I don't know if your furnace manufacturer provides a spot where there is an access panel in the outer wall of the hot air jacket so you can get to the inner firebox wall or not.. this is what is needed to safely install a hot water coil in the firebox.
When I took a tour of Keystoker's factory in June, I looked at a Koker furnace being built.. Keystoker has a designed-in location for hot water coil instalation into the firebox. This is inside the hot air jacket, and there is an access door or panel on the hot air jacket for installing the plumbing.
Hope this helps.. Greg L