A search of the literature for "kava" and "anaphylaxis" or "anaphylactic" turns up very little:
scholar.google.com
The only specific case reports that brings up are articles that mention various herbals which if you read closely all appear to say that there have been some cases of anaphylaxis associated with
echinacea, but no cases with kava. A PubMed search gives similar results.
A search for "kava" and the synonymous term "allergic shock" does turn up this one paper, which is the only mention of this I am able to find:
Schulze, Johannes, Walter Raasch, and Claus-Peter Siegers. "Toxicity of kava pyrones, drug safety and precautions–a case study." Phytomedicine 10 (2003): 68-73.
It mentions 2 adverse reaction reports of "allergic shock" associated with "kava pyrone" between 1986-2001 in Germany and Switzerland (see Table 1) It's very important to note that the "kava pyrone" that these adverse reactions was reported with would have been the low quality extract products that were in use in Europe prior to 2002. It's also important to keep in mind that these reports only mean there was an association in two cases, they do not prove causation.