Oh, thanks Myron. I'm sorry but even after looking at the code i don't get what it should do. Would you explain please? :-)
After playing around with many different plugins, i found that
https://www.dokuwiki.org/plugin:selectsearch useful. It adds a dropdown-list with configureable entries in front of the searchfield. The configuration is done in DW config as a list of namespace/displaytext entries.
Unfortunately it only supports positive/including namespaces (the ones starting with '@'). To realize a search which excludes a namespace (as starting with '^') i need to modify the source.
Another drawback of this plugin is, that it removes every namespace-syntax the user enters in the search. But i think that's okay, because users are still too lazy and dumb to use them ;-)
There is only one thing whats missing: A feature to search in the current namespace and below only. Maybe call it "Search from here" or so. Tomorrow i try to implement this. Maybe the code needs some rewriting, but since it is not maintained for 5 years it may be a good idea to adopt the plugin.