Ideally I want to hit a key combo and start typing the name of a tag while a dialog box shows a list of matching tags I can select from (a bit like the tag box when you ask a question on this site :). Using a mouse makes it quite a bit slower when working through lots of photos ...
And I am aware that in digikam 2.x I can apply a keyboard shortcut to a single tag, but that doesn't really do what I want. It just applies that tag to the currently selected photos - I can't just type and search in the subfolders.
Eventually I found this blog article describing efficient ways to tag photos in digikam.
To make it easier to find, here is their "method B" which allows you to use the keyboard only across your entire range of tags:
I could also add that one option you will be given while typing is to add the tag under the currently selected tag. Perfect :)