I've heard rumours of U.S. immigration/customs agents at YYZ somehow observing pax disembarking from Cuba flights (with the unofficial assistance of some Canadian customs/immigration people?) and then watching out for these pax who then subsequently preclear.

What I'd do, if I was an U.S. citizen, would be to book the cheapest all-inclusive in Mexico that I could find, just toss that whole stay, and fly to Cuba from Mexico (paying cash for the Mexico-Cuba tickets).

That might be a bit dangerous to do. It depends on how the flights are ticketed. You could be in trouble if say, you book on Cubana and the tickets are made as a charge from Cubana rather than the TA. Similarly, the ticket number could end up on your CC and could be traced if the government has it in for you.