I've seen the 040 Modifying Agency, and have often though 'wouldn't it be
nice if there was a MARC plugin in the framework to automate filling that
field with the librarians name'. Maybe someone should add a feature
request to bugzilla for that.

Yep!

A better solution would be file a new bug and store that kind of data in a new SQL table or if you prefer store it in a local file like holding fileds in 9xx.

There are a reference of holdings data fields in koha wiki:
<http://wiki.koha-community.org/wiki/Holdings_data_fields_%289xx%29>

Regards

Atte,

Hector Castro

Universidad de El Salvador