__dict__s and types and maybe metaclasses...

[Adam Atlas]

Suppose I want to create a type (i.e. a new-style class via the usual
`class blah(...)` mechanism) but, during the process of creating the
type, I want to replace its __dict__ so I can override some behaviors
during the initial assignment of its members. That is, I have `class
blah(...): a = 123; b = 456; ...`, and I want to substitute my own
dict subclass which will thus receive __setitem__(a, 123),
__setitem__(b, 456), and so on.

Is this possible? Maybe with metaclasses? I've experimented with them
a bit, but I haven't found any setup that works.