Following up to myself, I did the math for the following itinerary:

SFO-PHX / LAS-*-SJU-*-YUL-SFO

Looking up the cheapest one-ways I could find on Expedia regardless of who was offering them or if they earn miles, I got:
SFO-PHX $147
LAS-MIA-SJU $221
SJU-IAD-YUL $316
YUL-SFO $368
For a total of $1052 (taxes included). As expected, the transborder legs are the expensive ones.

Using the Airpass for all six segments (there are no direct *A LAS-SJU or SJU-YUL flights in April) plus two Caribbean surcharges, this works out to $1015 + taxes, so this doesn't pay off.

However, using the Airpass for SFO-PHX/LAS-IAD-YUL-SFO (4 segments), and a separate IAD-SJU roundtrip for $246 (taxes included), the cost is cut down to ($556 + taxes) + $246 = $802 plus some taxes.

So judicious use of the Airpass does seem to be the cheapest option, and I get the added bonuses of extra flexibility and guaranteed miles. Can anybody poke holes in my logic or suggest ways to squeeze out extra pennies? I'm almost entirely clueless when it comes to cheap flights in North America...