I can say with certainty it can be done...

Last year I was scheduled to fly YYC-YVR-SEA-YVR-YYC

for reasons too long to explain I wanted to drop the YVR-SEA segment.

I called to ask they said no because it would cancel the remaining segments.

I asked a gate agent at YYC they said the same thing.

I asked customer service at YYC they said the same.

I got got to YVR and asked a customer service agent and they were able to do it after not an insignificant amount of groveling.