O comentário de @ Stéphane Chazelas levou-me à resposta.
A falha foi de fato com o precmd
alias:
precmd echo '3]30;<my-host-name>precmd echo '3]30;<my-host-name>%pre%7\c'
7\c'
Unaliasing isto obviamente fez a linha ofensiva ir embora, e de lá mergulhar mais fundo era uma brisa.
De man csh
:
Automatic, periodic and timed events (+)
The beepcmd, cwdcmd, periodic, precmd, postcmd, and jobcmd Special aliases can be set, respectively, to execute commands when the shell wants to ring the bell, when the working directory changes, every tperiod minutes, before each prompt, before each command gets executed, after each command gets executed, and when a job is started or is brought into the foreground.
Obrigado.