As the original reporter on the PostGIS mailing list, I want to thank everyone both on that list and on the Postgres list for the very quick response.
Having the problem confirmed and a ticket opened in 2 minutes is really impressive. My report contained a very simplified version of our crashing query, but I've now also verified that the fix > - else if (arrayexpr) > + else if (arrayexpr && root != NULL) also stops our original big query from crashing. Thanks! /Fredrik