K-ballo: you're right as usual. if I replace
template <>
memory_region_allocator<unsigned char>::mempool_type*
    memory_region_allocator<unsigned char>::mempool_ptr = nullptr;

with

   template <typename T>
    typename memory_region_allocator<T>::mempool_type*
        memory_region_allocator<T>::mempool_ptr = nullptr;

then it seems to be happy - please accept my apologies for not quite seeing what you meant. I will use that for now, but I think longer term, tthe base allocator should be type independent. Thanks