🚀 go-pugleaf

Although not answering your question directly, you might be interested in
MMDF  II (which you sould be able to get from BRL in the near future),
which has a number of features to provide access control.  We (UCL) are
beginning to use these controls to restrict message flow between the
Internet and the UK.  Currently, if you attempt to loop a
message back to yourself thru UCL, you will receive a warning
message telling you to become authorised (for the UK -> US hop)!
This is done on the basis sender, recipient, and connect hosts,
to determine which networks may be accessed.

Steve