I'd like to install Prince 9 alongside 8.1 on a server, but there seems to be no way to specify where Prince looks for its data files (eg. hyph, dtd, style dirs).

You can pass the directory with the --prefix option on the command-line, normally this is passed by the shell script located in /usr/bin/prince to the main binary in /usr/lib/prince/bin/prince.

Thanks, that'll work!