Pretzel burger buns: how to make soft crust?
Since what you're making is a pretzel that you want to act like a soft loaf, I would try treating it like a bread.  

To keep the crust on homemade bread soft, people brush the top with a fat of some kind (usually butter).  So you could try liberally brushing the tops of your buns with butter when they're fresh from the oven and still hot.  That should keep the "crust" of your mini-loaf nice and soft.