Could you give an example of other utilities and players that provide dropdowns with all values so that you can apply those values to new, possibly untagged files?
What is your idea of a "custom tag"? A user-defined field? Which player or utility (besides foobar) can cope with user-definded fields?
How would you like to see the implementation if a collection has some 10.000 Artists and just as many albums? How long should the dropdown list be?
How should the list be maintained in respect to editing entries, deleting them?
Where do you see the increased user-friendliness in the thread that you linked
REQUEST: predefined disc field
where you have to scroll through 64 entries in a list just to enter 3 characters?