EWR764

YYZ is a preclearance airport, and all pax on a YYZ-ORD flight are already considered domestic so there are no customs issues.

If the diverted airplane parks on a gate, the passenger does not have checked baggage, and ground staff from the airline (and/or contractor) are available and willing to offload passengers, both physically and in the system in advance of the continuing segment, then it's theoretically possible. IME it's more likely in late night scenarios where ground staff aren't busy dealing with other scheduled flights and the diversion point is reasonably close to the original destination. In those cases, you usually have several passengers asking to be let off, and I find agents are more willing under those circumstances.

I wouldn't bank on it, in any case.