This has been discussed by
PMario a while ago. Personally, I don't see that as a functionality of
TagSearchPlugin. Instead, what you want in order for a consistent experience is to
always retrieve those tagging tiddlers in your custom order whenever a plugin or the core asks for them, i.e. also in the sidebar.
So what is needed is some code that hijacks store.getTaggedTiddlers() and returns them based on some sort criteria rather than the current core mechanism for that. I imagine there could be a order.tagging field at the tiddler being the tag holding the information of all tiddlers that actually have been ordered, with all new and unordered ones being put at the end.
There could be some UI in the EditTemplate. For starters, this field could be all manual and populated with current tags when opening the tag in EditMode. This means, that at the point where you want a custom order, your tag definitely needs to become a tiddler and not just be a tag.
For example you the UI could show a number next to each tag...
[X] Tagging Order (Enable / Disable)
[1] Foo
[2] Bar
[3] Baz
Changing the number for Bar from 2 to 1 would resort the list and save the order.tagging field as follows...
Saving changes to this field could be generally ignored so long as you have not modified it yet, i.e. you don't care about defining a custom order. So, hitting the enable / disable button would not only toggle a panel showing the numbering but essentially remove the order.tagging field unless you specifically want it.
Anyhow, this code is out of scope for TagSearchPlugin and should be something for a new plugin perhaps called TagOrderPlugin.
Extending this functionality to tags via some order.tags field (and not only order.tagging) would also remedy the fact that, due to indexing purposes, tags in TiddlySpace are always stored alphabetically, which sometimes really is not what you want / need... and actually broke some of the now outdated behaviour of ColoredLinksPlugin (being replaced by PaintrPlugin).