But unless you batch and re-order rays (and I'm not convinced doing this is worth it if you're doing more than 3/4 bounces, as the incoherence just makes things a nightmare), there's no point really doing this (unless you're happy with very slow performance - even mmapping stuff is slow).
Ray re-ordering is indeed a nightmare, but sorting very large ray batches (millions of rays) into coherent ray streams is less of a hassle, and it should enable coherent geometry access (which amortizes the cost of those reads, not sure to what extent).
For stuff like hair where you need loads of scattering events or for SSS doing that (paging subsections of shapes) is just going to thrash any geometry cache you have unless they're quite big.
And I'm still not convinced by ray sorting: it means your light integration is extremely segmented into artificial progressions as you expand down the ray tree, and makes anything regarding manifold exploration (MLT or MNEE) close to impossible to do well.
They do mention some tests with "out-of-core rendering of scenes with massive amounts of geometry" in the paper, but there isn't much info on it. AFAIK their patents mention streaming in geometry too.
I know that buying more RAM is probably easier, but it would be interesting to render very large scenes on commodity hardware. I guess paging in geometry is just too much of a hassle, you constantly have to stream in geometry to memory; to compute surface derivates, to do subsurface scattering, to sample arbitrary geometry lights, etc.