The original idea isn't good, but the guard town job sounds great, so let's talk about that again.
As for the incentive; builders got xp for building, although you could say "working for the greater good should be reward enough". Higher duel lvl means you are defending against tougher opponents = more xp/cash.
Obviously this would be queued just like any other job, and would require certain skills; Tactics, riding, shooting, or something. Whatever would work well with patrol and defense.
The risk rating would be an undefined variable, since this would entirely rely on whom visits town. The guarding soldier would be able to intercept incoming duelers, and thus protecting workers from getting preyed upon.
Shouldn't be 100% chance of blocking incomings though, as this would be an effective duel block, but depending on skills used for the job, and the amount of labour points, the chances of blocking ought to increase. This said, simply blocking wouldn't do much good for a min/maxed "town blocker", since he must also be able to win and survive the attacker, to fend him off.
Question then would be; how many duels should he be able to block in say each 10/30/60/120 min period? Too many, and he dies. Tough luck some would say, and in a way, I agree to that.
As for the original idea; I don't think giving +100% class feature is feasable. This would further label classes, and with the premium feature wich does the same.... we'd have utter imbalance on our hands.
How about just making it so that you can NOT level up to lvl 11 as greenhorn?
You will simply NEVER get that last digit xp UNTIL you choose a class.