I have an idea but you may not like it.
I would first try and procure a new slam latch before doing what I suggest. If you can buy one then start drilling out the locking portion of the old latch. See if you can access the release catch. If not than drill out the rest of the slam latch. Once the door can be open replace the old slam latch with the new latch.
Or call a lock-smith to see if they can get the latch to release.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67