Just came across this post over at RetroRoleplaying and am definitely considering it for a sword and sorcery style B/X game. While the basic background of the article is to allow for wizards and thieves to wear armor, the end result is that a fighter can get away with wearing magic chainmail instead of platemail because the AC difference is a lot lower.

The key points of the article article are reproduced below (mostly as a reminder to myself and in case the source page ever disappears) – refer to the source (linked above) for the full rules.

Base Armor Class: Each class has a base armor class:

Fighting Man — Base AC of 5
Paladin/Ranger/Monk — Base AC of 6
Cleric/Druid/Bard — Base AC of 7
Thief/Assassin — Base AC of 8
Magic-User/Illusionist — Base AC of 9

Armor:

Leather Armor: +1 to AC. Magic-Users and Illusionists cannot cast their highest level of spells known while wearing Leather Armor.

Chainmail Armor: +2 to AC. Magic-Users and Illusionists cannot cast their two highest levels of spells known while wearing Chainmail. Thief abilities are halved while wearing Chainmail.

Plate Armor: +3 to AC. Magic-Users and Illusionists cannot cast their three highest levels of spells known while wearing Plate Armor. Thief abilities are unusable while wearing Plate Armor.

Shield: +1 to AC, only when character is concious and mobile. Magic-Users and Illusionists cannot cast their highest level of spells known using a shield — if they are using a shield and armor tthe shield adds 1 to the levels of spells they cannot use.