Its late here (Sydney) so I can only give some quick tips.
At tiddlywiki.com search for filter operators and have a look at the search operator and if your game using regex.
Also remember you can next $list widgets so the outercould search for tiddlers with text and the inner could filter out those without your desired tag so you apply two different filters.
<$list
<$list
</$list>
</$list>
When you are more expierenced you may be able to combine them into a single filter.
Eg [[first filter]] -[tag[tagname]
I hope this helps
Tony
@Mark S
thanks that worked. As this is a quite a basic search, but useful with other filters (not only i the standard search) I think this should at least be included in the drop-down suggestions in the filer search. This operator was unknown to me despite I have used TW5 for years.