If you have a cuirass the arms show up even though it is not in the hands slot, so the problem has to be somewhere else. Since you know about the naming conventions (http://www.gamesas.com/bgsforums/index.php?showtopic=1074627&hl=arms) I'm not sure what the problem is. What slot do you want to use? Maybe it doesn't work on some slots (like the head slot for example - would make sense).
ahh yeap, did some testing
They will show up in first person if in the upperbody slot. If i put them in like the lowerbody they dont show up though. Ideally i'd like this in the tail slot.....