Hm, why don't you just do this:

with stat_tarfile.open(fileobj = sys.stdout, mode = "w|") as tar:
    for number in xrange(100):
        fileobj = generate_file_content(number)
        tarinfo = tar.gettarinfo(fileobj=open("/etc/passwd")) 
        tarinfo.name = 'file-%d.txt' % number
        tarinfo.size = len(str(number)) * 100
        tarinfo.uid = 1000
        tarinfo.gid = 1000
        tarinfo.uname = "dstromberg"
        tarinfo.gname = "dstromberg"
        tar.addfile(tarinfo, fileobj)

Wouldn't that work, too? Or am I missing something?