I would be careful with waltzing in to parks or city people with questions like this. They might morph something else they think applies to your "pressing question" (like, about disturbing earthworms or whatever) and saying "no". I don't know about any NFS place handing out "permits", so I can't comment on your exact thing. But I just know that md'ing on NFS land is not forbidden. In fact, there's an express allowance (can't argue with that, eh?).
http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5261774.pdf
Print that out if you're skittish, and have it with you. But if it's abandoned and un-used as you say, then .... is anyone really there in the first place to care ?

In any case, it's not dis-allowed.