The situation is that you wish to fly USA-YYZ-USB? And $JOB will pay for USA-YYZ; you want to pay out of pocket YYZ-USB and then eUp to get some sun and free $13/btl wine on the way?
$JOB may be willing to pay for USA-USB direct, which likely is cheaper then a TB flight. And then you can pay for an upgrade on UA directly, which may be less than the YYZ-USB hop.
I'd at least ask around for the travel rules (I'm in theory allowed to fly to 3rd party locations if its <~ the same $ as home, which anywhere else in NA is), and crunch the numbers for yourself.