Adding my own metadata into METS file

[Ruairi]

Hello!

In my pipeline, we have a form for archivists to generate DC metadata fields that would ideally be integrated into the METS file in the AIP. It's held in a JSON file inside the package being converted to AIP at the top level inside. The majority of my archivematica modifications exist in the codebase when the AIP is transferring to storage service, but at this point the AIP is already compressed and the METs file can't be changed at this point.

If I want to write a little code that lets me append my own fields to the METs file, right when the METs file is generated during processing, but before the AIP is done being created, where in the code could I do this/intercept?

Many thanks,

Ruairi