There is as far as I know no daemon interface or similar to manipulate the access control list for MercuryS or reload the ACL file. Another approach could perhaps be to write an event daemon to log relevant AUTH request data. 

Hi,

how can I dynamically enable MercuryS Session logging for a single IP address (and/or blocking for a single address)? I don't mind to disable it manually later.

Why?
We're under some SMTP AUTH attacks (up to 2.500 per try).
Whereas Mercurys hopefully just blocks these tries the lack of forensic logging (each single attempt only produces a line "T 20160418 130122 570a35ad AUTH LOGIN") makes it easy for abuse depts to "don't believe" a report.
That's why I would like to enable session logging for such addresses as soon as it starts to happen. Manually it can be done using config | MercuryS | connection control.

I can just
echo 8 2.3.4.5 >> C:\Mercury\MERCURYS.ACL
but how do I make Mercury recognize this config change without killing mercury.exe (loader loads it new)?

Thanks!