Once you rule that plate is not available (except as penalty-carrying bronze breastplates), heavy armor become much less appealing. Players will generally want AT-20 or AT-1. Unless you make AT-12 available, which I would take in a second. The question in Rolemaster has generally been how to make armor attractive rather than the reverse. I wouldn't bother with house rules. Maybe double the price of what armor is available due to rarity.
However, the idea that avoiding armor is Conanesque is simply false. Conan certainly wore mail. The nigh-naked warrior, male or female, is largely the product of cover artists. In the Hyborean age, those areas with sufficient metallurgy were mailed, and it was certainly an age of iron.
Here is the very first description of Conan I grabbed when I started looking for text to make this point: "The stranger was clad like himself in regard to boots and breeks, though the latter were of silk instead of leather. But he wore a sleaveless hauberk of dark mesh-mail in place of a tunic, and a helmet perched on his black mane. That helmet held the other's gaze; it was without a crest, but adorned by short bull's horns. No civilized hand ever forged that head-piece. Nor was the face below it that of a civilized man: dark, scarred, with smoldering blue eyes, it was a face as untamed as the primordial forest which formed its background. The man held a broadsword in his right hand, and the edge was smeared with crimson." ("Beyond the Black River", REH)
Heck, a quick page through gets me Conan endorsing the value of armor, and adding a clear statement of the maneuver penalty's effect: "Conan tapped his mail-shirt and helmet. 'If more borderers would wear harness there'd be fewer skulls handing on the altar-huts. But most men make noise if they wear armor.[']"
