@russss I'm quite familiar with both Modbus and 4-20mA stuff and I don't get how this would work at all.
Maybe a Modbus to HART over 4-20mA and back again with something doing appropriate access control in the middle. My memory of the HART system is old and I'm not sure it can support access control, although I guess it could have been cobbled on in the 25 years since I last dealt with it but it all sounds insane.