Hello Johan,
I tried to reply but it didn't post to the group- I'll try to remember what I wrote and re-type it now!
The concept of extensions is coincidentally one that we've been discussing recently at Artefactual. It's a way to "extend without modify" meaning that important new functionality can be used in AtoM but without the same weight of technical debt. We'd love to collaborate with you on this further, so we can make extensions something that can be more easily contributed and used in the AtoM community. We will be in touch!
Cheers,
Sarah