I haven’t kept up with all replies here, but I have made the following additional feature available at quick-demo:
Populate the tag-override-filter field, at a tag tiddler, so that the tag-pill can display some list other than the default [<currentTiddler>tagging[]] (Generally this will be a list starting with or based on the basic list of tag-childer, but it may add other nodes, or possibly leaving some tagged tiddlers out, as needed.)
Unlike the basic idea through most of this thread, this solution is specifically intended to include virtual nodes that would otherwise be impossible to list within a tag pill.
For example, a tagpill for fields-for-bibtex could include ALL fieldnames that start with bibtex- including fieldnames for which there is no tiddler.
Obviously, this is specifically useful for wikis where there is some kind of virtual-node apparatus for doing important things with fieldname nodes (such as showing an overview of field:value pairs for tiddlers that have the field in question).
There are various ways in which tag functionality is deeply designed around expecting only actual tiddlers to be listed in the tagpill dropdown. So this solution is a work in progress, intended (as is much of this work!) as a proof-of-concept.
EDIT TO ADD: This additional development overlaps (again!) with the work Tones has been doing on “flags”… But here I’m again sticking more closely to the idea of working with the tag tiddler (and much of its role in the core) so that it can be adjusted or tinkered-with to handle different demands.