My transmitter has a BPF in its final RF stage...
Any LC tuner is a kind of a BPF, for sure. However, I had a follow-up BPF between the tuner and antenna in mind.
...its harmonics if any are way above 100MHz (200, 300, 400 etc).
There are other kinds of spurious signals other than harmonics of a carrier and its sidebands. These don't necessarily have any relation to the carrier frequency and/or the harmonics of the modulating signal. Such spurs could come within the aircraft VHF band. Have you checked this possibility? Furthermore, spurious response may not even originate from a circuit defect. Overmodulation can cause all sorts of unwanted radiation. This could have caused "splatter" that could have generated interferance. After all, that circuit of yours isn't just an FM modulator. It's an AM modulator as well.
If all is well, then, obviously the Canadian "RF Cops" are actually doing their jobs. May be you should consider relocating to the USA? It's incredible what the FCC will let you get away with. :roll:
So long as your name isn't "Howard Stern", that is. :lol: