Hi Derfel,
Am afraid I have not tried it without a password protected macro but it should work with the built in command "Macro" "Load New Macro File".
To check to see if there is a newer macro file available at startup, I would make a playable macro file (make the macro in the editor, then right click it in Macro Explorer and click Export - Export as playable macro).
To actually perform the check, one method that may work (I do not think Macro Exp actually has the ability to check when a file was created) is to have a text file in your network path with the latest macro version.
Inside that text file would be the latest version number of your macros. Similarly, each machine would have a text file containing its current version of macros. Then by putting the two values of the text files into variable and comparing them, if they do not match then run the "Macro" "Load New Macro File" command. (First I would copy over the latest macro file to a set directory on the computer though).
Put a shortcut to this macro into startup and then everytime the machine is started it will run this macro and perform the check and update your macros to the latest version.
Sorry for the long winded reply - hope it helps you a bit! (if it even works at all!!)
Ells