The design looks great. Easy to grasp and covers almost all imaginable use cases.
'Almost' because it would be nice to also have ScopeAllocator!N ~ FallbackAllocator!(InsituRegion!N, Mallocator) which would automatically deallocate all heap-allocated blocks when exiting the scope (i.e. no deallocate method, array of pointers to be freed is kept on a C heap).
