AHA! OK, I discovered www/doc/rank_for_search.sql. Despite being an OpenACS addition, it doesn't seem to be referenced by the OpenACS documentation.
Anyway, if after adding the postgresql-tcl RPM, I do
su - postgres -c "createlang pltcl acs"
psql -e -U postgres acs acs < /var/lib/aolserver/server/defaultacs/www/doc/rank_for_search.sql
then it works fine. So no special RPMs are needed, just postgresql-tcl as created by Lamar Owen, and then the two commands above (adjust for your database name and PG user (both are acs in my case) and the location of your OpenACS tree).
Clearly, it would be nice if this process were documented somewhere. Maybe in the OpenACS Getting Started Guide would work?
Roberto: Is that the right place to do this? Or should it be considered an optional installation step, and therefore go in the OpenACS Installation Guide instead? Once I know, I'll have a go at a patch for the relevant document. Also, I'll patch ad.tcl to add a couple of comment lines just above the OpenACSSearchP setting.
That was instructive, I just learnt a bit more about debugging OpenACS
Thinking slightly longer term, it would be neat if the OpenACS RPMs could set a trigger on the presence/absence of the postgresql-tcl RPM, and adjust things accordingly... this way bboard searches would "just work" if you had postgresql-tcl installed.