templatedirs => [ "my/dir", "/usr/share/ikiwiki/templates" ]
ought to do the trick.
+
+> Now that I look at the source, I see that ikiwiki already falls back to the
+> global dir when it cannot find a template. For me, this is good enough.
+> And it is even documented in the man page. Sigh. I guess this could be
+> considered [[done]].