So, why not write a mod_perl module that will do just that (restrict user powers)? ISPs could then configure their servers to force using mod_perl powers via this module only. Users gain speed, performance, DB persistance, etc; ISPs gain control.

I'm still learning mod_perl, but from what I read, I think something like this is very possible. Most of the users don't need the full mod_perl horse power, just a small, but still significant, subset. Module like this could enable it, I believe, exporting some API and handlers users should then use instead of default ones, and controling that default stuff doesn't get called.