[Oscar Benjamin email@example.com]
... I don't believe that a standards-compliant decimal module has any wiggle room to invent a new kind of rounding (which I think would be required to achieve what you suggest).
That would be true if the standard explicitly forbade extensions (any optional operation or behavior not specified by the standard). But there are no standards like that ;-) To comply with a standard, there just needs to be _some_ way to "spell" all the behaviors the standard mandates. For example, in a programming language no relevant 754-ish standard says that the infix binary operator "" has to implement 754-ish multiplication. So far as standard compliance goes, "" could mean "ignore the second argument and add 1.7 to the first argument, rounding toward 7" - just so long as there's some way to spell "754-ish multiplication".
Python (or any other implementation) is quite free to invent all the rounding modes (or any other kinds of operations, or modifications to standard behaviors) it likes. That kind of experimentation is where ideas for the _next_ iteration of the standard comes from.