If that doesn't work, I would recommend looking into <xf:set> - that should point you in the right direction
If you search through templates.xml for example usages of xf:set, you may find a solution. I've not tested it myself, though, so I'm just guessing here!
Haven't tested it, but if I recall correctly, something like {{ phrase('by_user_x', {'name': username_link($article.Thread.User, true)}) }} should work.