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?
Cheers