I don't know where I originally saw it, but I've been curious about it since I first ran into it. From my memory it's roughly this:

"All IS units are considered to have CASE by default in locations where appropriate. (Torsos for mechs, etc.)

Any IS Units that normally comes with CASE, instead has CASE II in that location.

Clan units have CASE II in all locations that they would have normal CASE protection.

That's as best as I remember it; I don't recall if the original post I saw accounted for CASE II, and I might be wrong about how Clan units are handled. Either way, I believe the original context I saw this for was specifically for 3025 era or Clan Invasion era games.

Regardless, any thoughts on this house rule? What does it break? I initially liked the idea a lot as a way to mitigate the silliness of, say, assault mechs with machine guns getting blown up just because they didn't squat down and take a dump of their missiles just prior to the fight.