I think you are walking really close to the edge. You might be even over with this solution. I guess most of folks try to stay close to standard especially when we are talking about security. Or at least they should. So chance of finding somebody who used this and survived is really slim IMO.
Regarding S4/HANA I think it's too early to say if merging all systems (ECC, CRM) is a good idea and customers are going to adopt it. Even if they adopt it the IT landscapes are much more complicated and you should start looking into implementing IdM solution. SAP is also pushing for hybrid approach (cloud with on premise). For this you definitely need a dedicated system that manages identities. Centralization from security point is a good idea but I don't think it's feasible in todays world.
Cheers