I had the same problem, hacked up a little python to do it for me.  I must say it was a pain, s3 is a total freakshow.
https://raw.github.com/reklis/utilityscripts/master/github-upload
#!/opt/local/bin/python2.7
import json
import requests
import sys
import argparse
import os
import mimetypes
import pycurl
import cStringIO
from xml.dom import minidom
github_api_root = "https://api.github.com/"
def parse_args():
    parser = argparse.ArgumentParser(description='post a file to github as a download')
    parser.add_argument('--user', dest='user', help='github username', required=True)
    parser.add_argument('--pass', dest='password', help='github password', required=True)
    parser.add_argument('--repo', dest='repo', help='the name of the github repo', required=True)
    parser.add_argument('--file', dest='filepath', help='path of the local file to upload', required=True)
    parser.add_argument('--desc', dest='description', help='descriptive text about this file', required=True)
    parser.add_argument('--owner', dest='owner', help='owner of the github repository', required=True)
    args = parser.parse_args()
    # print args
    return args
def make_dl_post_url(owner, repo):
    url = "%srepos/%s/%s/downloads" % (str(github_api_root), str(owner), str(repo))
    # print url
    return url
def make_dl_delete_url(owner, repo, dlid):
    url = "%srepos/%s/%s/downloads/%s" % (str(github_api_root), str(owner), str(repo), str(dlid))
    # print url
    return url
def add_github_reference(args):
    dl_post_url = make_dl_post_url(args.owner, args.repo)
    fp = args.filepath
    filename = os.path.basename(fp)
    filesize = os.path.getsize(fp)
    mtype, mdetails = mimetypes.guess_type(fp)
    file_description = {
        'name': filename,
        'size': filesize,
        'description': args.description,
        'content_type': mtype
    }
    # print json.dumps(file_description, indent=2)
    github = requests.post(dl_post_url, auth=(args.user, args.password), data=json.dumps(file_description))
    resp = github.json
    # print json.dumps(resp, indent=2)
    return resp
def remove_github_reference(args, dlid):
    dl_delete_url = make_dl_delete_url(args.owner, args.repo, dlid)
    github = requests.delete(dl_delete_url, auth=(args.user, args.password))
    delete_ok = (204 == github.status_code)
    return delete_ok
def post_file_to_s3(file_path, gh):
    # s3 is very particular with field ordering
    # curl \
    # -F "key=downloads/octocat/Hello-World/new_file.jpg" \
    # -F "acl=public-read" \
    # -F "success_action_status=201" \
    # -F "Filename=new_file.jpg" \
    # -F "AWSAccessKeyId=1ABCDEF..." \
    # -F "Policy=ewogIC..." \
    # -F "Signature=mwnF..." \
    # -F "Content-Type=image/jpeg" \
    # -F "file=@new_file.jpg" \
    # https://github.s3.amazonaws.com/
    s3_ok = 201
    xml_buffer = cStringIO.StringIO()
    try:
        post_fields = [
            ('key', str(gh['path'])),
            ('acl', str(gh['acl'])),
            ('success_action_status', str(s3_ok)),
            ('Filename', str(gh['name'])),
            ('AWSAccessKeyId', str(gh['accesskeyid'])),
            ('Policy', str(gh['policy'])),
            ('Signature', str(gh['signature'])),
            ('Content-Type', str(gh['mime_type'])),
          ('file', (pycurl.FORM_FILE, file_path))
        ]
        # print post_fields
        s3 = pycurl.Curl()
        s3.setopt(pycurl.SSL_VERIFYPEER, 0)   
        s3.setopt(pycurl.SSL_VERIFYHOST, 0)
        s3.setopt(pycurl.POST, 1)
        s3.setopt(pycurl.URL, str(gh['s3_url']))
        s3.setopt(pycurl.HTTPPOST, post_fields)
        # s3.setopt(pycurl.VERBOSE, 1)
        # accumulate string response
        s3.setopt(pycurl.WRITEFUNCTION, xml_buffer.write)
        s3.perform()
        file_upload_success = (s3_ok == s3.getinfo(pycurl.HTTP_CODE))
        xml_payload = minidom.parseString(xml_buffer.getvalue())
        if (file_upload_success):
            location_element = xml_payload.getElementsByTagName('Location')
            print location_element[0].firstChild.nodeValue
        else:
            print xml_payload.toprettyxml()
    except Exception, e:
        print e
        file_upload_success = False
    finally:
        s3.close()
    return file_upload_success
def main():
    mimetypes.init()
    args = parse_args()
    # step 1: tell github about the file
    gh = add_github_reference(args)
    # step 2: upload file to s3
    if ('errors' in gh):
        print json.dumps(gh, indent=2)
    else:
        file_upload_success = post_file_to_s3(args.filepath, gh)
        # cleanup if upload failed
        if (False == file_upload_success):
            removed_ok = remove_github_reference(args, gh['id'])
            if (removed_ok):
                print "removed github reference"
            else:
                print "failed to remove github reference"
if __name__ == '__main__':
    main()