Figure or text outside a block in tikzposter

Yes, you can: In tikzposter you are already in a big tikzpicture. Just place your nodes as usual. The center of the page is (0,0).

One can also mention that the class defines a coordinate bottomleft in the lower left corner and topright in the upper right, so these can be used to place stuff relative to corners.

\documentclass{tikzposter} 
\begin{document}

\node [above right,font=\Huge] at (bottomleft) {LOWER LEFT};

\node [above left,font=\Huge,outer sep=2cm] at (bottomleft -| topright) {LOWER RIGHT};

\node [below right,font=\Huge] at (bottomleft |- topright) {UPPER LEFT};

\node [below left,font=\Huge] at (topright) {UPPER RIGHT};

\end{document}