We're happy to hear you're planning to create packages that make use of nimble. As you suggest, I think the most helpful thing might be to point you to some of the existing packages that depend on nimble (e.g., nimbleEcology, nimbleSMC, nimbleSCR, BayesNSGP). You could take a look at their source to see how they have handled things.
While dealing with export/namespace issues is in my court on the nimble development team, I'm not remembering at the moment what needs to be exported in this case. After you look at how those other packages have done things, if you see behavior that seems non-standard/suboptimal, please let us know and we can look into it in detail. One of our current goals is to make it straightforward to create packages that depend on nimble, so we want to hear about any difficulties.