Is it possible to have/use a shared custom field (i.e. entity checkbox) across multiple SuiteApps?

are you asking from an SDF perspective? can it not just be the same object in multiple apps?

That's what I'm asking. I haven't tried it yet but from my experience so far I think it would conflict because the app id would be different on the field. I think you would have to add shared objects via script to accomplish this.