x2  Reclaiming the heat is an attractive idea but this is one place where sticking to code might be best.  I had the uninfortunate experience of owning a condo unit, and being on the board of directors during the time that it was discovered that 75% of the dryers in the building had been retrofit installed and vented into the units.  Several just dumped into into wall cavitys, both practices being against code in Ontario.  Most had been running this way for 20+ years.  The mould problems behind the walls, around windows, and in other hidden places througout the units were extreme.  Years of moisture collecting and pooling on the window sills rotted them and the surrounding structure.  The mould and structure damage was very costly to fix.  Of course this is an example of extreme neglect and your milage may vary.