Instead of (or rather, extending) the global mining level option, allow better CADs to mine one level above the set level. I suggest basing this on potency to allow for addon CADs.

Or, alternatively, set it so the global mining level option is diamond by default, and set iron/gold/psimetal CADs to do 1 level lower than default. Mathematically the same result, but it allows the config to have the actual maximum.

Another way would be to actually do what Belgabor said not to do, and extend the global mining level option to include options for separate CADs, while somehow letting addons set mining levels for their CADs as well (maybe just by having the addons set their mining levels in their own configs, so you don't have to deal with it as much).