I haven't been following Meteor too closely lately, but from the start it implemented minimongo on the client side, which is a good subset of the Meteor+MongoDB integration. This allows Meteor apps to mostly be 'offline by default.' Of course, Meteor makes many other decisions for you which you may or may not like, but I think they've been trying to pick out their functionality as libraries, so maybe there's a low hanging fruit there w.r.t. getting it to work offline without adhering to the whole framework.