If you make the end YYZ-DAY (does AC fly that? If not, YYZ-ORD-DAY?), then simply no-show the last segment(s), that should be legal to book since it's no longer open jaw. There's a risk that, in IRROPs, they'd re-route you to DAY rather than YYZ, but lots of people take that risk all the time.

The idea of buying a cheap cash SIN-DPS-SIN seems good though, provided you can build in enough connection time.