I still don't particularly like the idea of the assert_* methods returning something.

If the call args tuples had args and kwargs attributes, for which there are outstanding feature requests, then you could simply do:

my_mock(1, someobj(), bar=someotherobj())

foo = my_mock.call_args.args[1]
bar = my_mock.call_args.kwargs['bar']

By avoiding the extra step of tuple unpacking this is still nice and readable (IMO).