Sorry for the silence:  new country + new job + no internet connection 
outside work = not many opportunities to spend time on this (or any 
other part of Python) at the moment.

I do now have a version 0.2 of the deccoeff type, and more importantly a 
version of decimal.py that's adapted to use it;  all tests in 
test_decimal.py pass, which is a start.  But there's significant work to 
be done tidying up the code, identifying speed-critical bits, and moving 
those bits from Python to C.

I'm wondering where to post the code;  I could post a tarball here but 
it's a bit unwieldy.  Perhaps it would be worth creating a new branch to 
work on this?

This definitely seems like a (>=) two-person task;  any help would be 
much appreciated!  Thanks Facundo and Nick for comments so far.  I also 
wonder whether there are alternative approaches that might do better.

If we follow this approach, I'm doubtful about doing this for 3.0.1; 
even though the API is unchanged, this seems like too big a change for a 
bugfix release, with too great a risk of breakage.