OK, let me make my hypothetical itinerary really extreme, so you'll see what I'm getting at:
Monday: LAX-YVR AC555 Dep 1545 Arr 1845
Tuesday: YVR-YUL AC112 Dep 1415 Arr 2155
Wednesday: YUL-YYZ AC427 Dep 2000 Arr 2120
Thursday: YYZ-LHR AC848 Dep 2020 Arr 0835
(This isn't actually a valid itinerary, though there are itins that allow this many stops)
Could one stretch a single SWU to upgrade on all 4 flights, since each of the stops is less than 24h? And even it were possible in theory, wouldn't the agents at YYZ on Thursday get a bit shirty on being told that the cert was uplifted at LAX on Monday?