This would probably be a little too powerful, for the cost of a feat (Armor Proficiency: Chainmail) it would grant classes that normally use Leather or Hide armor a +4 - +8 to AC (depending on level), most feats that boost AC seems to usually only give a +1 or +2.
In general classes that use light armor are able to have an adequate AC with some combination of Int/Dex modifiers and/or class features that boost AC. Changing Eladrin Armor to count as light armor would allow these classes to still get those bonuses while also getting the increased AC of Chainmail.
For reference these are the bonus Leather, Hide and Chainmail give at various levels:
Leather
1-15 +2
16-25 +3
25-30 +4
Hide
1-15 +3
16-25 +4
25-30 +5
Chain
1-10 +6
11-15 +8
16-20 +9
21-25 +10
26-30 +12