Here's my workflow with mp3tag: https://www.youtube.com/watch?v=JrXekn8N_PE
Unfortunately, for per song metadata, I have to go through each one by one (mouse gesture is down). It is very fast for classical and single artist albums, but huge box sets can be a chore. Unfortunately, I haven't figured out how to have mp3tag do a "walk" where the same command is executed for each song down the list w/o confirmation messages because each song already has a unique AMG_SONG_ID that directs it to the AMG page for the song (see above).
EDIT: For the "walk", autohotkey might be the answer, but damn I have to dive into its documentation since I'm not familiar with its syntax.
EDIT 2: Found a promising post on stackoverflow that might lead me to the answer: https://stackoverflow.com/questions/15946630/autoencode-in-notepad/22990842#22990842
EDIT 3: Okay, here's an autohotkey script (Win+S) I just made that performs a walk down a tracklist in mp3tag running a web source for each track automatically. To quit it, press Win+Q. Basically, it prompts the user to enter the number of tracks in the album (enter about 1.5X more). Then it runs the following key sequence: alt+s (this brings up the websource menu), l, t, enter, enter, down (next track). For example, this will run the following named web source script per track down the tracklist: &Last.fm#&1 &Track metadata.src. This works best if the song already has a unique refID that bring up the song page immediately. Thanks for motivating me to find solution to this particular use case.
#q::
BreakLoop = 1
return
#s::
#IfWinActive ahk_exe Mp3tag.exe
InputBox, times, Number of tracks (more than on album)
if (times > 0); test if it is a number
{
Loop % times
{
if (BreakLoop = 1)
break
Send !s; alt+s to bring up tag sources menu
Send l
Send t
Send {Enter}
Send {Enter}
Sleep 1000
Send {Down}
} return
}