Then I find that alone makes it worth investigating. (Make that one more item on my long list of ideas..) Most ready-to-install scripts come as mod_perl-friendly CGIs, with the main appeal of mod_perl besides the acceleration of the scripts themselves being persistent database connections. Just being able to run those in a shared environment would be a big boost.
On the other hand, how much work would it be to offer a stripped down mod_perl that only offers enough of the API as to be sandboxable like PHP? Idly mulling, it seems it might even work to strip down mod_perl via httpd.conf rather than at compile time.
Makeshifts last the longest.