Based on your description, I'd say go for it. They've given you the money to AUP, and you can probably survive the few cases you can't due to a full cabin.
My current client is strictly Y only as well (to a certain distance) I book WTP and AuP into CW as suits. I swallow the costs, but really regard them as part of the contract, i.e. I negotiated price with that in mind.
Good luck whatever you choose, nice dilemma
Edit: for clarification, I expense the Y fare.