I would go for something where, say, _foo was a system run level, and
a namespace reserved for system magic.  Future inits could add extra
meaning to such levels.  foo would be a user-defined run level.  Why?
So init can know what it's doing better.

Apart from that, why limit them?  If you have too many run levels, your
system becomes slower and init gets too big.  That's at ~30k run levels.
I wouldn't worry.

-s