Hi,
If you have a look at Tobi Beers tagSearch plugin. It may be an option.
In TagSearchConfig you can define tag "categories", where only one tag of a category can be active.
eg:
fruits
apple
orange
So, if you have a tiddler tagged apple and you want to tag it orange there will be a dialog, if you want to remove the apple tag. This is not exactly what you described, but I think it would be worth experimenting.
Also have a look at zzConfig tiddler (first line) to activate the plugin configuration.
-m
http://tagsearch.tiddlyspot.com/#TagSearchConfig%20Action