Yeah you could use a 76-79 era RM Profab/DG, it is same/correct length but you will have to make adaptions for the DR swingarm pivot which is larger than the RM. Not much to gain with it really because it wont have the banana bend it like the DR ones. You gotta watch out for bends/axle alignment probs with the Profabs too. I have been caught out a couple times with one axle slot higher than the other, therefore axle is tilted up/down to one side
The DR Profab/white Bros arms might show up on ebay once every 10 years if you are lucky unlike the Yamaha ones where its on average evry 3 or 4 months. So they are very rare, just not many were ever made. By the time they were making the DR400/500 arms they were using larger box section than the older TT and RM Profabs too.
Like i said the stock DR500 Z/D/E alloy arm is good (apart from weak/thin pivot tube like the PE's - watch for cracks/splits here) so i dont see why you would want to use a KLX arm that will require some mods.