The fields that would be editable would be configurable
On first read, I didn't get that these would be fully configurable to the users' needs - with that in mind, this suggestion does become more interesting to me as the main tags I access on a regular basis do live on tab-2 (Tag (2) ) of the main editor and are a bit cumbersome to reach (Hit Enter to open tag editor -> click "Tag (2)" tab -> click field one wants to edit -> add/change value - which could be much more streamlined imo).
So this could work for me as a nice feature if this gets me quick-er access to around 7-10 custom tags...
The one thing that does worry me for Stephen, that this could be dev time spend on features that aren't going to be used, is that quite often I do need multi-file tag access for tag editing; and IF I only need to do single file tag edits, I think I still prefer the in-line editing capabilities in column view even in its' current somewhat mixed state...
Which leads me to:
Current in-line edit functionality is right there at your fingertips when you need to do quick tag edits on the fly and is fine for my needs as is.If there was a way to quickly toggle the 'enable direct editing of tags in the main panel' preference, then this would be my preferred way for single track tag editing.
+1 for the quick toggle 'enable direct editing of tags in the main panel' suggestion, but I've said that elsewhere a few times as I would really love for MB to be easily able to jump between its' 2 main 'states' i.e. a edit & a non-edit mode (including disabling the star rating click-ability!).