ZDoom

This is technically possible via ZScript menu stuff - see the secret map in Reelism 2 for an example - but it requires a bit of faffing about and also requires you meddle with ZScript menu stuff, which, to quote the friend that wrote the code, "is like waking up from a dream where you die and then you immediately die in real life". I think it also has an issue where if the menu has been initialized pre-unlock the unlock won't show until the game is restarted... not sure how to fix that one in any graceful way. That's probably on us, though.

This seems already achievable, with a map, through which, the player can access the various episodes. Since it's a map, you can do some scripting to block access to the bonus/special episode until the player has achieved the requirements for it.

Say I wanted to make a one off "episode" in the vein of Resident Evil 2's The 4th Survivor (or later games Mercenaries mode) that requires the player to complete an episode...or all the episodes first (which simply would execute a custom cvar when pressing an exit switch before the end level handling starts) or alternately, someone wants to make the episodes properly sequential (i,e they have to complete episode 1 before they can start episode 2)

Essentially GetCvar would function as other GetCvar counterparts, if the operator number is successful to the result, the episode selection appears, if the operator number is less, return false. etc.