Which naturally sets $PACKAGESITE in the environment for all users.
In this case, you may want the environment variable set for all users that
login, but you don't want it leaked to services for various reasons (in the
OP's case, there may be nothing that can be done about enumerating the
environment -- it may be a required setup -- but you don't want this variable
to give away pertinent security-specific information that could facilitate
hacking your machine by knowing which version of the OS is in-use).
The default value for the proposed new knob would be NO.
The knob would be opt-in only and on a per-service basis.
It would act as value-add on top of existing features like above.
As for your stated options (a, b, c list), I concur with that list.
I would add that as long as the rc.d script uses the rc.subr routines for
starting services according to rc.conf settings (descriptive of the fcgiwrap
rc.d script), then the new knob would be applied regardless of whether you use
service or invoke the rc.d script manually.