How to draw a gate with tikz-uml

Question

I would like to be able to draw a "gate" (described here) on a sequence diagram, but I have not been able to figure out how to do that with tikz-uml. The closest I've been able to come is to use a \umlboundary, as shown in the code below:

\documentclass{report}
\usepackage{hyperref}
\usepackage{tikz-uml}
\begin{document}
    \begin{tikzpicture}
        \begin{umlseqdiag}
            \umlboundary[no ddots]{EXT}
            \umlobject[no ddots]{A}
            \umlobject[no ddots]{B}
            \umlobject[no ddots]{C}
            \begin{umlcall}[op=external]{EXT}{A}
            \end{umlcall}
            \begin{umlcall}[op=query, type=synchron, return=identify]
                {A}{B}
            \end{umlcall}
            \begin{umlcall}[op=hb, type=synchron, return=register]
                {B}{C}
            \end{umlcall}
        \end{umlseqdiag}
    \end{tikzpicture}
\end{document}

This results in the following diagram:

However, I don't want the circle, "EXT" label, lifeline, or rectangle associated with the umlboundary object, I want it to look something like this:

Is there some way to draw a gate with tikz-uml?