Writing a plugin to add "target COM" to the AGU when engaged with debris

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I was away from KSP for a while, but since I've been back, I've come to find that one can no longer target the COM of a piece of debris once latched on to it with the AGU. I seem to remember being able to do this back in the day, just as I can today with asteroids. This seems so backwards, as it's the engineered object whose COM should be able to be known, while an asteroid's should be less certain.

I've considered options like a "pull design" tug that could perhaps work, but I'd still need to know roughly where the COM is at the time of attachment, so that's not my ideal. Also, I just feel like I should be able to know the COM of an engineered object (even if the engineers were kerbals). 

I'm open to other ideas and suggestions, but at this point I'm wondering what would be involved in writing a plugin that revives this capability. I imagine the devs had a reason for removing it, so maybe there is some big barrier to success here, but it seems like it might be worth a try... 

I've written up contract packs and waypoint collections in the past, and have a bit of "backend" experience, so something (seemingly) simple like this seems like it would be within my capacities. I'm just not too sure where to begin. Any help would be appreciated!