Hi Piers,
This is a request we've heard before and has certainly been discussed. There are no public plans to tackle this as part of Firebase at present that I can share.
However, if you or any other readers want to take a little time to detail the specific use case you'd like to solve here, which isn't solved by Firebase's current offline offering, I'll be sure your feedback is discussed by the appropriate gurus. Feedback has and will continue to be a pivotal part of Firebase's roadmap, so I appreciate any time you spend helping us understand how you'd use this.
☼, Kato