There does not seem to be any guidance available for installing QuestDB on FreeBSD. An enquiry on Slack created helpful suggestions, but nothing directly actionable.
For posteriority I decided to create my own installer. This seems to do the trick, and is modelling on the Homebrew configuration, for lack of a better idea.
As root:
pkg install bash
curl -LO https://github.com/questdb/questdb/releases/download/6.0.4/questdb-6.0.4-rt-freebsd-amd64.tar.gz
tar xvf questdb-6.0.4-rt-freebsd-amd64.tar.gz
cd questdb-6.0.4-rt-freebsd-amd64