Preventing line break in a displayquote environment

In the text I am writing I make extensive use of blockquotes, which I need to refer back to at a later point (kind of like numbered theorems, really). I use displayquote for this purpose, and it works pretty much as expected. (The referring part is also taken care of, using label)

documentclass[fontsize=11pt,BCOR=8mm]{scrartcl}
usepackage[main=ngerman]{babel}

usepackage[silent]{fontspec}                   setmainfont{Minion Pro}
usepackage{setspace}                           
usepackage[german=quotes]{csquotes}            

usepackage{etoolbox}                           newcounter{quote}

    AtBeginEnvironment{displayquote}{refstepcounter{quote}singlespacevspace{-.5em}smallitshape}
    AtEndEnvironment{displayquote}    {hfillrlap{scriptsizenormalfont[thequote]}vspace{-.5em}endsinglespace}

begin{document}

A bit of dummy text here to get us started, followed by the first quote:

begin{displayquote}
Bei [Gastmann] ist das Böse nicht der Ausdruck einer Philosophie oder eines Triebes, sondern seiner Freiheit: der Freiheit des Nichts. (S.~109) end{displayquote}

Not an issue; the quote is formatted as desired, and the counter is displayed.

begin{displayquote}Als jahrelanger Gesandter Argentiniens in China genieße er das Vertrauen der fremden Macht und als ehemaliger Verwaltungspräsident des Blechtrusts jenes der Industriellen.~(S.~73f.)
end{displayquote}

In this case, however, [2] is shown in the next line. This must be prevented, even at the cost of bleeding into the right margin.

end{document}

If I have a longer quote that fills the last line, however, my quote counter is shown in the next line, all by itself. I would like to prevent that from happening.

I am using LuaTeX, by the way. Not using fontspec seems to solve this issue, but is not an option for obvious reasons.