Introversion

Cyan is correct. There should be a surrender option.

The lack thereof is particularly glaring in CTS. Multiwinia should allow eliminations on CTS when the opponent has no spawns and no men (and trails the leader by at least one capture).

that would promote rage-quitting

I disagree. There is no such thing as 'rage-surrender'. If you are going to concede the game, surrendering would be more polite than leaving abruptly without comment.

there would need to be conditions for surrender to be available, and they would need to be close to the defeat conditions

It depends on how the game is processed afterwards for ranking purposes If it is just listed, surrendering any time would not be a problem. Likewise if the only thing entering ranks is whether the game was won/lost (for 3+ player games: the first to surrender is considered losing the most). Only if the exact scores would count, surrendering could be problematic.