Perhaps when an undergeared player enters a duty and is paired up with players who are correctly or overgeared, maybe the system should recognise the difference in ilvl between the players and will pop up with a message similar to what you get with FATEs - "Your item level is lower than the party average and your contribution will not be weighed as heavily". Or alternatively the duty would have an ilvl which is recommended but not enforced, and a similar message would highlight the player about this.

Of course it wouldn't in any way penalise that player if they are undergeared, but least they know on entry the degree of difficulty or how much/little they can handle. If the party chooses to leave, abandon or kick the player then that's completely up to them though if something does go wrong, then the player should have an idea on how to improve for next time.