changing the envmap column in appearance.2da will also make EVERY armour and robe use CM_Baremetal, even the ones that are supposed to use CM_Bright (light blue glow), or any other envmaps for any specific armour.
I think you were on a better track by using the .txi file, but you probably just had it named wrong. I can't remember off hand if it is should be named the model's name or the default texture's name (example PMBN.txi or PMBN01.txi). I'm not at home ATM, so I can't look up the exact names you'd need.
The problem with .txi files for robes or armours, is that it will change ALL the envmap for that one specific armour or robe type. It's not so much a problem for robes though, since none of the vanilla ones have any alpha channel, nor do they have an envmap. It only becomes a problem with modded robes. For instance, Darth Darkus' Armored Robes (
http://www.lucasforums.com/showthread.php?t=202234)don't) use CM_Baremetal, but there are two that have ragged edges (he used the alpha channel to make part of the 'cape' transparent). When I made some shiny metal versions of those (with permission for RoR), I couldn't keep the ragged edge, since it started shining like metal instead.