There is in fact a YUL-UVF service (though it's via MIA with a short stopover) and can be booked as a single flight. I originally had this in my DONE4 (though we had to waitlist for D which AA granted in approx. 10 days) but I found the UVF-LGW service limited my options in Europe without surface sectoring to LHR so I dumped it.

But to the OP, YUL-UVF can definitely be done, as can LAX-YUL which is not considered a NA transcon, unlike LAX-YYZ.