Serial registration for both?
For AoW3 at the start of a game each player has to check with Triumph's server that will then authorize them for that play session. "PBEM" would presumably be per turn. Needs Triumph account. DEFCON
, AFAIK at each launch, checks with the dev's server to see if the serial is still valid. If not it switches the game to demo mode. Not that I would own this, but I don't recall if the Linux installer was ever updated with an always offline unlocked version. Account based via serial key.
What the problem with Neverwinter Nights?
Last time I tried playing it in LAN mode, it needed one authentic key each for base game and each expansion, I will add it as soon as someone can confirm that this has changed or it does work with GOG's stub serials.
Are you redefining what LAN mode is? NWN1 multiplayer is direct IP with dedicated servers. Works LAN, Online and, IIRC, even has updated master browser servers. Anyone with a valid copy can play LAN, GOG finally changed to automated serials for this and, AFAIK, the person running the server can set serial check.