Eric,
The office manager at Swainson Safaris(I forget his name) has details of the top canoe safari operater who does these trips on the Zambezi.
Mana pools to Chewore would be a really good trip. I imagine the crocs at Mana pools and the Hippos at Sapi(for some reason they appear a bit aggressive at Sapi) can be a slight problem, but an experienced guide will have no problems.
I actually met a group of pensioners (average age 70 yrs) who were on a canoe trip from Kariba dam to Kenyemba ( Swainsons).
Go for it, and let us know how you got on.
M.