I've been struggling with the varnished interior/oiled exterior for W/C canoes as long as I've been hanging around this forum. While in my 20s, I was "schooled" in cabinetry/furniture building; we built high-end stuff: curved exterior balconies, arched entries, roll top desks, frame & panel walls, fauncy fireplace surrounds, etc. I learned that any finish applied to one side of the wood absolutely must be applied to the other side of the board, to prevent differential expansion situations due to fluctuating humidity.
Many years later, after I'd left the trade, I started building paddles, and I was faced with the situation of laminating woods for blades, where the woods and grain directions being used were listed in Bruce Hoadley's "Understanding Wood" text as having very different expansion characteristics, due to changes in humidity. Indeed, I found that the laminates that ended up in the grips of my paddles behaved differently over a year's time than the identical laminates (same board, same grain) in the blades... expansion lines became evident after less than one year in the grips, but not the blades... the grips were oiled, the blades were 'glassed & varnished. Some smart guy told me to use laminating epoxy (e.g., West System 205 Resin/207 Hardener; other brands will work fine) as the first finish coat (no 'glass; just epoxy). This seems to have completely eliminated the expansion lines, in the grips.
What this would seem to indicate is that there may be some advantage to varnishing the exterior of the not yet canvassed canoe, just as you would the interior...
I'll note that this is theoretical; I have yet to restore a W/C canoe. The 1918/19 Old Town HW in my shop is awaiting my retirement. I'm really curious about this issue, but have no real experience with it. Thoughts of more experienced builders/restorers would be appreciated!