I can't help you with any ideas of my own, but I give credit to you and your group for looking for ways to take your fiancee' into the BW in spite of his wheelchair.  You're a special lady.

If he needs any assistance sitting in the canoe, there are adaptive seats being made by Kevin Carr at Chosen Valley Canoe Assessories.  Here's the website:  

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Give him a call to see if the seat might work in your fiancee's situation.