there is no way to get the data in the original into the derived copy except by doing it by hand
1-1 or 1-many:
(on the file menu: plugin extras) which brings up:
If you want to do "many to many-others-like-them", you have to use a different plugin, like Relative Antics..
If I understand it correctly, "Custom File Info panels" for xmp data should just solve this problem.
Many of us are hoping Adobe enhances Lr to provide better support for custom metadata - as you've noticed, Lightroom is not one of those "Standard Adobe Applications" referred to in the above link.
So, you are saying "Please read the manual"? Understood, and I will but i'm not sure that either of your methods are better than the Adobe Sync Metadata menu item (which is what I meant by "doing it by hand").
What I do currently is isolate all the images that should have the same value for your custom metadata fields, select 1, then select all (leaving the first "most selected") then hit Sync Metadata, and I already have the correct fields checked from the last time I did it. This is easy in my current workflow as I already have all the images that should have the same value in the Quick Collection.
FWIW, I tried to automate this process using a keyboard macro program (Keyboard Maestro), but Lightroom is somehow written that if the keystrokes/mouseclicks come too fast, then it does something different than intended (like sync from the not-most-selected to the most-selected), and slowing down the macro to make it reliable means that I can type faster than the macro.
I will look into Relative Antics.
Should one file a bug report that Lightroom is not a Standard Adobe Application?
What I do currently is isolate all the images that should have the same value for your custom metadata fields, select 1, then select all (leaving the first "most selected") then hit Sync Metadata
I thought you meant: typing them in by hand.
So the problem is that it's not done automatically when you're round tripping to an external editor.. - got it.
Relative Antics does have an automated/background mode which could detect when a sync is in order and just do it.., but "some assembly required".
PS - I don't think the omission in Lr constitutes a bug, but there is a feature request for it (or two) in the feedback site, you may be able to find if you look:
Ciao for now,