Probably make the thirst work like hunger. Make sure the integration is fully optional!

https://www.curseforge.com/minecraft/mc-mods/dehydration

Edit: there is now an API https://github.com/Globox1997/Dehydration

I've decided to give up on supporting non-vanilla mechanics because I have no interest in using them personally and any logic changes must be constantly maintained.

Modders are free to mixin their mod's support into OneBar instead, like some have already done.