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A pickle subset ("gherkin"?) could possibly also fill this
need, if it was documented, even though pickle format is
considerably more complicated than marshal format (it uses
marshal.dumps for binary output, actually taking apart the
marshalled strings).  It was obvious in seconds how
marshal.c works but after 30 minutes of looking at pickle.py
I'm still not sure I understand it.  It looks like
the unpickler can construct arbitrary class instances and
import arbitrary modules, which makes a security hole
if the pickled strings are potentially hostile, but
I might not be reading it right.  Also, the unpickler
must implement constant folding (the memo scheme), which
complicates it somewhat, though it's not that bad.

The idea of leaving the marshal formats of some Python-
specific objects undocumented isn't to get out of
documenting stuff, but to leave those formats open to later
change.  

Re BER/DER, Burt Kaliski's "Layman's Guide" is pretty
readable (http://borg.isc.ucsb.edu/aka/Auth/ASN1layman.htm).
You're right about using all 8 bits in BER integers--it
looks like the 7 bit representation is only used for OID
components (I didn't realize that til checking on it just
now).  BER might be ok for what I'm doing--I'm not sure
right now since I don't understand ASN1 that well.  It looks
not in the spirit of marshal/pickle though: to encode a
compound object it looks like you need an ASN1 spec of
EXACTLY what you expect to find in the object.