The problem with grabbing information from the 'additional info' portion of the page is that it's simply one giant text field, and not an easily read and standardized list like the standard track-listing page.
For example, for him to modify the script to pull track listings from the additional notes, he would have to program it to discern what the difference is between a track title, and another block of words in that same field. I'm sure there's a way where it could look for numbers beside titles, i.e. 12. Grim Dark Death Grimness, to figure out what the title is, but I think that's beyond the realm of what a script like this can do without serious overhauling, or complete recreation.
And that's not even considering additional info notes regarding multiple bonus releases with the same track number, but different songs, from separate international releases. For example:
12. Grim Dark Death Grimness
12. Dark Grim Forest of Eternal Darkness
It's just not feasible again because of how Metal-Archives has set up the additional notes as just one giant text field.
I know how you feel though, I had the same issue.
My solution was to just take the last track of the 'official' track list, click 'tag copy' and then 'tag paste' to every bonus track. Then, it just takes a couple seconds to change the track number (highlight all the tracks, at the top of the program you'll see a little button with numbers that will auto-number it, or change them one at a time manually starting at the beginning of the bonus tracks), and I just typed in the bonus track titles individually. Usually it's no more than a track or two, so it wasn't too much of a hassle.
That being what it is, as I said, I know how you feel, so I've thought about this as well. What I would suggest is trying to change Metal-Archives in the way they list bonus tracks. For example, either within the main track list page, it would be nice if you could add a 'bonus tracks' sub-heading to put stuff in, similar to how they separate 'disc 1' and 'disc 2', from large releases (example here), or even simply have bonus tracks have their own separate tab on the album, much like how they have additional notes/lineup/reviews, etc.
Long-story short, you're asking the wrong team for the catalytic change.
For your other point, I believe the reason that Year and Publisher are left alone is because of how programs read the ID tags on the tracks. For example, I can sort my MP3's in my program by year, and I know of some where you can search by Publisher, and I'm certain that adding a METAL ARCHIVES YEAR/PUBLISHER, could mess up how these programs pull information from the MP3. In reality, the entirety of the METAL ARCHIVES - tags are just for fun, because really all you need for a proper functional collection are: album, artist, genre, publisher, title, track, year (and album art, in my opinion). So, I guess consider the MA tags simply as fun little bonuses as opposed to pertinent information regarding the release. If it bothers you, you can always remove them just select the track, hitting Alt+t, and then highlight all the MA tags, and click the little "X" on the right and it'll leave you just with a uniformly standard set of tags. I know I've thought about that so my metal/non-metal releases all look the same, but I think the bonus info is too fun to get rid of.
I think your idea about tagging the country code somewhere would be neat. I know I have a few bands with the same name that I have to tag oddly to differentiate them. I don't know about converting countries to abbreviations (seems like a lot of work) abut a METAL ARCHIVES COUNTRY tag would've been neat.
The only thing I really wished for was that it would pull the genre. Man, I really wish that could've been implemented. I know nothing about coding, so I'm not sure how difficult that would've been, but it certainly would've saved me a lot of manpower.
To be honest, there's even a couple of things I would have removed (and still might from my collection). I don't think the METAL ARCHIVES DATE is entirely necessary as long as the year is correct (but perhaps it's important to some), and given the nature of the reviews, having the METAL ARCHIVES RATING on there is really only valid at the moment you do it. Someone could come along and add a review that changes that number as soon as you're done. I do quite enjoy the METAL ARCHIVES TYPE tag though.
Anyway. I've rambled.
I've been tagging MP3's so much that I literally dream of it. From the time you see my first post in this thread, until now, I've just finished tagging my (ever growing) metal collection of 1002 bands, 3208 albums and 30434 songs, and I couldn't have done it without this amazing program and dano's top-notch script.