I did a week-long externship at San Antonio and it was awesome. Very beneficial to know what a post-grad program is like. They didn't charge me anything for the externship, but I did have to fly there and find housing for the week.
If you google "endo externship" you'll find info on these two programs who allow externs. That's how I found them.
Best of luck. When I get to Maryland in July I'll post if we would let you come out as well.