Thanks for the thoughts everyone.
The heat exchanger Gary provided is intriguing since the exhaust air side could be cleaned easier since that air wouldn't be running through the individual Coroplast tubes. Great for a HVR, but the lent filter requirement (I agree its a huge issue Daox) for it still concerns me. As a result, I'm still leaning towards a co-axial heat exchanger since it would require only cleaning out a single pipe with a brush (or hopefully water hose) to remove the lent. I know the efficiency would be lower, especially if the exhaust pipe doesn't have some way to add turbulence and break up the laminar airflow but I'm willing to take that hit in exchange for not needing a lint filter and worrying about excessive back-pressure on an already long vent run.
What I had in mind was a co-axial heat exchanger 16 feet long, with a 4" AL inner tube and a 6-8" insulated flex-duct outer tube. That would give a surface area of ~2400 sq. in or 1.55m2. over which to conduct the heat through thin-walled AL. A quick look online shows that a basic overall heat transfer coefficient for AL in an air heat exchanger (Forced Convection (flowing) Gas - Condensing Vapor Water) is 10-50W/m2K. So for a 42K temp differential (thanks for the basic numbers above Gary) I could expect anywhere from ~650W to 3.2kW of energy transfer.
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