Does a halonium ion for astatine exist? I've just begun reading about halonium ions and I noted that only the first four halogens are acknowledged to be able to form halonium ions, but hydrogen astatide can be formed.

I can't seem to find any answers on the internet, but knowing that astatine is super reactive and radioactive, I suppose it may be due to this that forming an "Astatonium" molecule would be nearly impossible? 

On the Wikipedia entry of hydrogen astatide, it states that there is an Astatonium — listed as a conjugate acid. However, a quick Google search of the term Astatonium only reveals 14 results.
No astatonium ion with hydrogen is known, but it appears that a cation $\ce{S7At^+}$ is formed by replacing a sulfur atom in $\ce{S8}$ with an isoelectronic $\ce{At^+}$ moiety (the actual oxidation state of astatine, with sulfur being more electronegative, is $+3$).  The ion is proposed to explain the incorporation of astatine into the sulfur precipitate when a sodium thiosulfate solution is decomposed by acid in the presence of astatide ion.

Primary reference:

G. M. W. Visser, E. L. Diemer, Radiochimica Acta 33 145 (1983).