WikiVoyage has a small but clever extension to do quicker lookup of
pages in additional content namespaces if no namespace prefix is
provided. So if we used this for Commons, and configured "Category:"
as an extra lookup namespace, you could type "Low quality chemical
diagrams" and it'd pull up the category of that name immediately
without the full-text search being loaded. 


Thoughts? Would this be a useful improvement for Commons in
particular, even without autocompletion? I suspect it could help
reduce some of the category/gallery pain.

This sounds like a very good addition, yes.