The problem there is that 2.4's exception statements and 3.0's have two different meanings.

2.4 only supports this:

    try:
        do stuff
    except FooError, e:
        do stuff with e

3.0 and up choke on this; they interpret it as "catch exceptions of type FooError or e and do not give the exception object a name".

2.5 through 2.7 interpret like 2.4 but also support explicit parentheses to do what Python 3 does:

    try:
        do stuff
    except (FooError, BarError) as e:
        do stuff
    except (BazError, QuxError):
        do other stuff

2to3 requires that you use the new exception syntax only, to avoid confusion.

That is precisely my goal, I think it's eminently feasible using 2to3.