Currently, TORRegionSpec is defined using two *const u8 pointers, start and end, however the end pointer is more like "one-past-end" because it is being written as is (with shifting by 2) to pmpaddr register. It means that this mechanism can't be used when the uppermost region needs to be defined. That is, when end is one past the 0xFFFF_FFFF address on a 32-bit system.
Similar problem exists when we want to have a single NAPOT region to cover the whole memory space (for instance when we want a "default" PMP permission for everything). In that case the region is defined by it's start address and a size expressed as usize. But usize can't express the full size of 32-bit space.