One of our bigger Foreman environment is used to deploy Ubuntu clients. These client can only be identified based on their MAC address because we have no control over the DHCP client subnet.
Currently Foreman only supports MAC-based provisioning for CentOS/RHEL by parsing the appropriate Kickstart HTTP Header.

The clients are deployed using iPXE which already fetches its chain script via Foreman. Meaning we have no other option (e.g. token) than the MAC address to identify the clients.
As soon as the iPXE chain script has been fetched from Foreman we can again make use of the build tokens (e.g. to access the rendered preseed template).

There was already a similar PR (https://github.com/theforeman/foreman/pull/1106) but it was never merged. Currently we run Foreman 1.9 with a custom patch which allows us the provisioning based on MAC addresses.
Due to OS restrictions we currently cannot update Foreman but this will change in the next few weeks when we will migrate to the latest and greatest version. Therefore we'd really like to have this feature included
into the next version to prevent any glue code. :)