Just cart it onto the plane with you, and sound like trouble if you are challenged in the doorway. I do that with a 120 bass accordion, plead that I am a musician, and am never parted from my instrument. I just look suitably crazed, and, so far, I haven't had to get stroppy. There seems not to be any communication between plane and booking in desk, so just do it. They can't get access to the hold by that stage, and it's basically a problem they have to solve by finding a space somewhere in the passenger bit of the plane.

It's also worth trying the "They said it was alright at the booking in desk" line.

Good luck

Skipjack