Convince professor to allow submitting paper?

There's some work X that I did with my advisor. We got stuck, and she thought it was too preliminary to submit for publication. She didn't think it was worth putting the effort to turn it into a submission. Now we're on to some other work Y. I'd like to submit X for a journal/conference, since it can't really hurt to submit. If anything, we might get useful feedback on future directions from referees.

What are some things I can do/say to convince my advisor to allow me to submit the work with her? For one thing, I'll tell her that I'm happy to do the writing up.