There have been a bunch of threads recently which have mentioned harvesters being treated differently to non-harvesters. (Things like how players 'con' and honour points jump to mind.) People have rarely continued to think through the ramifications of their ideas though. They seem to give up at the start saying, "the game can't tell who is a harvester and who isn't".
The client knows if I have a pick in my hand - it lets me do prospect/harvest actions if I do and it doesn't if I don't.
The game knows whether I've joined in group or solo combat or not - I get xp if I have and I don't if I haven't.
Think through your ideas again and don't give up (some were good, some were awful

). There is no reason why the game can't use existing mechanisms to label a character 'non-combatant' or 'harvester'.