OBD-II, while it makes more sense than OBD-I, is pickier about how things are done. You can get codes for things that don't cause a problem - a good example would be using a separate regulator for the alternator instead of the inbuilt PCM regulator circuit (not common, but some people do it that way in the course of a swap. Or, convert to an internally-regulated and/or "one-wire" Delco unit.)
Yep - throws a code. Nope - can't get rid of it. And yet, you don't have any driveability problems. The code doesn't so much come from low system voltage (that's not what's monitored,) but the fact that the PCM doesn't see anything happening with the regulator signal. Oops.
It takes a little more thinking to modify OBD-II systems than pre-OBD, but that's about it. I have a distinct preference for pre-OBD - simply because it's easier to modify and use "parts on hand" to effect repairs without the system going into "b**ch mode" or "limp home mode" just because it doesn't like what I did.
Hazel is probably going to go deeper into mods than you intend to - and OBD-II will likely get in his way. OBD-I is also likely to get in his way, and he'd be better off with a mid-1970's rig (beginnings of electronic controls, pre-OBD, easy to modify) instead of OBD control setups that check everything - whether you want them to or not.
The problem with persistent OBD codes is that the CEL/MIL stays lit, and that can result in a genuine problem being "covered" by the persistent non-problem that you know about (you won't see the CEL/MIL light up, because it's already lit for no good reason.) Since you can't programme OBD to "ignore" specific faults, that gets in the way of some modders.