Thank you Peke!
I put a few tracks together, available for 7 days and sent the link via ticket 12517.
1. Yes, it shows as RELEASETYPE (album), RELEASESTATUS (official), CATALOGNUMBER (...), MEDIA (CD) as extended tags in file properties.
2/3. Generally, I run them through Picard before they ever hit the library to clean up the tags, match it to a release, and make things "perfect". Then I drop them into the folder for MM to pick up.
As an example in reply # 9 at
https://getmusicbee.com/forum/index.php?topic=1001.0, you can see that Musicbee maps the custom tags to individual tags in different file types. This allows the custom tags in the database to still be used like normal, but be read into things. I don't love this, as you can change it (and then should a database rescan overwrite the custom tags?), and I don't really need these custom tags elsewhere.
In a perfect world for a solution would be to add a mask field that references a specific extended tag if it exists in the file (I don't need this used anywhere else in the database). That would let me do pretty much everything i need, provided it can be used in an IF clause and displayed.
My end goal is:
/artists/singles/Artist - Single Name [ABC 12345] (2024)/...
/artists/singles/Artist - Single Name [ABC 54321] (2024)/...
/artists/albums/Artist - Album Name (2021)/...
/artists/albums/Artist - Unique Album (Deluxe 20th Anniversary) [CDP 12345]/...