Lands are only being counted at the end of the game, so you'll only have to do this once per game-session.
Since you'll know all completed cities beforehand (to calculate points for the cities), I'll probably do the following:
- Remove one "follower" from the list of followers (list A) that were placed on "land". Add the follower to list B.
- Start a flood-fill at the followers position.
- If your fill hits a city (which wasn't added before), add it to a list of adjacent cities.
- If your fill hits another follower, remove it from list A and add it to list B
- Separating road-pieces, rivers or missing pieces will obviously stop the flood fill.
After your flood-fill completes, count the number of followers from list B for each player. Depending on what ruleset (second edition, third edition) you'll have to implement, you can now split up points for all players that placed their followers in the land in question.
Now repeat this procedure, as long as list A still contains any followers.
I might be mistaken, but I guess one plug (or node) per corner should be sufficient to model the different possible connections.