Expanding newts
by HåvardA modified type of armour is perhaps even more interesting than a new feat. I agree that losing the leaping ability should remain in force.
Perhaps the armour could be something like this:
Newt Chainmail
This modified chainmail is created to accommodate the newt's special disdain for heavy and warm armour. It is lighter and allows for more air to help keep the Newt cool, even in combat situations. When worn, the Newt does not suffer his normal -4 penalty for wearing armour, but still loses his Leap Ability.
Perhaps combined with this feat:
New Feat: Newt Knight
Requirement: Newt, must have become accustomed to living among humans, wis 13+
You are a true Knight of Newts. You are able to overcome your disdain for armour. A Newt Knight does not suffer the normal -4 penalty for wearing metallic armour. If wearing special Newt Chainmail, he can even retain his Leap Ability.
Normal: Newts suffer a -4 penalty to combat rolls while wearing metallic armour, and lose their Leap Ability.