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Martin, you're right about add and sub, but it's a shallow 
assumption easy to relax (basically just declare 
carry/borrow as twodigits instead of digit).  I'd be more 
worried about the stwodigits type, but since nothing is 
actually broken here I'm not keen to fritter away time 
proving bounds on the temps in bigint division.  How we 
implement bigints internally is off topic anyway (provided 
we're not trying to hijack internal implementation formats 
for unintended purposes).

About BER, yes, and the URL I included is to a freely 
downloadable copy of the X.690 std; section 8.3 spells out 
the INTEGER rules.  They aren't at all the rules Paul 
sketched, hence "I don't really know what you [Paul] mean 
by 'BER'".

For the rest, while xmlrcplib may meet the letter of what 
Paul asked for at first, it's clear to me that it doesn't 
meet what he really wants.  My suggestion remains to add a 
new, efficient bigint format to pickle, which would meet 
everything except Paul's desire to have a special gimmick 
limited to his specific application and without having to 
write one himself.  The internal API functions 
_PyLong_AsByteArray and _PyLong_FromByteArray already do 
the heavy lifting in both directions (to or from base 256, 
unsigned or complemented, big- or little-endian).