If you think of a hosted solution, you can consider OAR to be just the "front-end" service which manages a library. That library (database and database contents). So as @roberto_barbera says, it's not possible to "skin" a single instance, but you can consider the library storage itself as a shared service, with several instances of invenio using it. They will all write to different databases with their own slightly different schema. If TERNET wants or is able to host such a service, they can provide a centralised web-facing cluster of OAR instances, and manage the DNS for each client university, and manage a centralised reliable db cluster for the individual libraries.
This is kind of the opposite of "mirroring", which folks like the ArXiv do, but could make sense in an environment where there is one good site and many unreliable ones where the users connect from. I don't think we're ever going to get to the point where we need a CDN-type distribution service to provide high-availability... people just use libraries that much , but centralising the resources and managing them (especially the naming system) as a service might make sense in a national or regional academic context.
I think a similar setup might be interesting to the WACREN or ekoKonnect people, @OwenIyoha_52c6, @Omo.Oaiya - what do you think ?