My only concern is that it be maintained, so that the OpenLDAP backend can be a supported part of Samba4 into the long term. (I know it's not been in good shape for the past while, but there is hope we can get it back on track).
Do contrib modules come with this level of maintenance?
Inherently no, because they would not be configured/built along with the rest of the package. Of course, as soon as users (possibly including myself) notice issues, they would be maintained (see for example nssov).
The most straightforward way of having it routinely built is to place it in slapd/overlays/; I guess this needs consensus from the rest of the team. I'd vote in favor; please holler.
p.