Bolted to the inside of the door skin metal is the only way to fly on armrests, IMO.
If it was me doing it.. i'd figure out what kind of armrest you were going to use (i.e. home made, one out of a later model vehicle, etc) and you'll probably find it gets attached with two bolts.
Figure out where you wanna mount it on the door, then mark your holes.. then drill the marks out with an approperate width bit.
I've heard of some dudes getting a cheap tap and die set and threading the door skin metal.. might be enough (depending on how fine or course your mounting bolt threads are) to get two threads or so there.. not what i'd like to see holding an arm rest on, tho!
My reccomendation would be to get a nut and epoxy it to the inside of the hole.. then when you get your new door skin on, you can just run the bolts thru the armrests, thru the door panel, and right into your nut and tighten 'em down (little loc-tite might not hurt, either!)
I'd use one of those nuts with the little lock washer thing attached to it.. that way as you're tightening the bolt you don't end up having the nut spin inside the door.
