Just an update; probably my final post unless any other suggestions surface.
Finally talked to a Canadian Customs guy, who was very accommodating and did a deep dig into their regs. Beyond the two bottle per person duty-free allowance, nothing on their books formally allows them to waive the full duty / taxes / LCBO fees payable on arrival, so any accommodation would be “at the discretion of the customs agent” at the port of entry – i.e. Toronto. Since getting the wrong agent could mean getting hit for as much as a $1000 tab, that’s not a gamble I’m eager to take.
Also talked to Air Canada, who had several ideas, all of them either equally costly or logistically impractical .
So it looks like we’ll be sucking it up and flying from the US. Oh, Canada!