if I'm not mistaken the portjail is basically working in it's own environment isn't it?
if so it would use it's own .cshrc and .login files
which would be inside the jail itself.
if so, simply modify .cshrc and .login like this:
alias sp set prompt='\[!\]PortJail\[$cwd\]:\ ' # set sp to set the prompt
alias cd 'chdir \!* ; sp' # redefine cd command
alias pd 'pushd \!* ; sp' # redefine pd command
alias pp 'popd \!* ; sp' # redefine pp command
sp # set the prompt
it would work mostly the same as the normal prompt, which a minor differance, normally the host-name is displayed, here the host name is replaced with "PortJail".
echo "Notice: you are using the PortJail!"
echo "The PortJail is only made to manage FreeBSD Ports and works only on a jail"
echo "Commands you use here will not affect the main system."
echo "Some commands will not be available."
and you have both portjail in the promt and a login message