We're working on this. It is not all that straightforward as it may seem, especially since MAC addresses are only visible in the immediate vicinity of your Pi-hole (it will not work across routers, VLANs, etc.).
You can monitor
https://github.com/pi-hole/FTL/pull/762
for our progress. Currently, it is still a feature draft, albeit a working one. If you (or anyone else) want to test it at any point, it would be appreciated.