Michael,
In both environments, the iBGP route-reflector should not change the next hop attribute, and the reason in both cases is the same. By having the RR change the next hop, the RR would necessarily be putting itself in the data plane. In most cases, it is preferable to have the RR act only in the control plane so the most efficient path can be taken.
The tricky part about BGP in a DMVPN environment is accounting for knowledge of the next hops by all BGP clients. As you know, in BGP, if an advertised route has an inaccessible next hop, BGP will not install the route in the RIB. The example in this Network Lesson was pretty simple since all spokes and the hub had outside addresses all in the same subnet. This will almost never be the case, which means you will likely need to advertise these next hops via an IGP via the DMVPN tunnel. This will allow each BGP client to know how to reach the advertised next hop.