Changing the tag-pill output will, of necessity, require modifying the macro. However, you can define a local variant of the tag-pill-inner() macro code, rather than changing the core definition.
My TiddlyTools TagCloud is an example of a modified tag macro:
In this code, I add a "tag count" to the tag-pill display. I did this by first copying the tag-pill-inner() definition from $:/core/macros/tag, and then adding display of the $(count)$ variable, the value of which was set by my code, before invoking the <<tag>> macro entrypoint. Here's my customized version of tag-pill-inner()
\define tag-pill-inner(tag,icon,colour,fallbackTarget,colourA,colourB,element-tag,element-attributes,actions)
<$vars foregroundColor=<<contrastcolour target:"""$colour$""" fallbackTarget:"""$fallbackTarget$""" colourA:"""$colourA$""" colourB:"""$colourB$""">> backgroundColor="""$colour$""">
<$element-tag$ $element-attributes$ class="tc-tag-label tc-btn-invisible" style=<<tag-pill-styles>>>
$actions$<$transclude tiddler="""$icon$"""/> <$view tiddler=<<__tag__>> field="title" format="text" /> ($(count)$)
</$element-tag$>
</$vars>
\end
For your example -- using an alternative label instead of the tag name ---, you could do something similar, by replacing the part of tag-pill-inner() code that displays the icon and tag name, like this:
\define tag-pill-inner(tag,icon,colour,fallbackTarget,colourA,colourB,element-tag,element-attributes,actions)
<$vars foregroundColor=<<contrastcolour target:"""$colour$""" fallbackTarget:"""$fallbackTarget$""" colourA:"""$colourA$""" colourB:"""$colourB$""">> backgroundColor="""$colour$""">
<$element-tag$ $element-attributes$ class="tc-tag-label tc-btn-invisible" style=<<tag-pill-styles>>>
$actions$<$text text="""$(label)$"""/>
</$element-tag$>
</$vars>
\end
Then, let's suppose you want to show the tag's caption text as the label...you would invoke the <<tag>> macro like this:
<$vars label={{!!caption}}><<tag>></$vars>
or, if you want to use some specific literal text:
<$vars label="argle bargle"><<tag>></$vars>
Note that the <<tag>> macro relies upon the value of <<currentTiddler>> to determine which tag is being rendered.
enjoy,
-e