Hey Jeremy and Epic, thank you for this solution. We implemented it, it’s simple and elegant!
It seems to have a side-effect, though, at least with 5.4.4: It breaks an unrelated RVT 
The system seems sensitive to the number of RVTs or RVT volumes or sharing of SVTs.
Our main landscape HLOD RVT + SVT situation is solved with the suggested approach, i.e. creating another volume (same size) with a new RVT, but with the shared SVT and the same number of mips as the shared SVT has. Perfect.
Elsewhere in the game, however, we have other RVT volumes, with other RVTs with their own SVTs. One of them breaks the moment with plug in our new shared RVT into the unrelated HLOD material.
Do your engineers perhaps have a recollection of a CL in 5.5+ that might be addressing this and that we might not have?
Given our time pressure, we might not be able to repro this in vanilla 5.4 in time. We can try, though, if this doesn’t ring a bell.
Thanks for additional thoughts, Pavel