If any of this is going to make rc.conf.local mandatory, then I don't
really think it is a good idea.

I would also have rc.conf check for rc.conf.local and source it if
it exists, rather than doing it in /etc/rc - that way everything that
reads rc.conf will get the local additions.

I would also tend to attempt to avoid the "config file turns into a
distribution file" syndrome - that's where a file (or directory) that
was created for users to set up their configuration, then gets distributed
with a sample configuration, and then people want to avoid touching
as otherwise the next system update with an updated sample will blow away
the local changes.  Then the once configuration file becomes an untouchable
data file, and a new file is needed for configurations, and the whole
thing repeats...

A better procedure is to simply come up with an update procedure that allows
the user updated file to be updated when the system is upgraded.  For rc.conf
that might be as simple as adding a few lines of comments between each line
that users are likely to edit, then applying judicous use of patch to make
updates (the comment lines serving as invariant delimiters for patch to match
against).

kre