Application Form 1093 to apply for a box....so you can fill it out in
advance:
http://www.usps.com/forms/_pdf/ps1093.pdf
The use of drop boxes for Appalachian Trail hikers is well documented but here's
a website that explains it better than I can.
http://www.appalachiantrail.org/hike/thru_hike/mail.html
(This might work as well as getting a full blown P.O. Box for BM but like
anything else if abused or if the postmaster gets swamped, it could come to an
end.). Anyway based on this it looks like using the U.S.P.S. in one way or
another is definitely an option.