In some games you can set a flag on the character to not show certain items (like never show helmet in NWN2), but unfortunately in BG/BG2 the only way to not show armoured appearance is to set a flag on each individual item, not the character. For example, if you didn't want to show your thief wearing Shadow Armour, you would edit the item and set the 'Equipped Appearance' to 'none' instead of 'Leather Armour' on the LEAT08.ITM. What does this mean? Well, you can do it yourself, but it would mean either altering every set of armour, in which case all members would be affected, or you could edit copies and give the 'none equipped appearance' copies to your protagonist via ShadowKeeper. A simpler way, of course, would be to create a single item that your protagonist will wear the entire game. Of course, this assumes you want to delve into editing/tweaking. If not, tweak packs are your only option.