Right now when you go to settings and click on the cell for a search engine under the keyword column, it shows a text box for me to add keywords to use in the URL bar. You can’t do so for bookmarks, tabs, or history which are locked to * % and ^ respectively.
If you can live with one character extra,
@t<tab>
also works for me.@b<tab>
lands me on Bing, but if you don’t use Bing you can just remove that search engine, I imagine.