The purpose of MappingProxyType is to provide a read-only proxy for mapping. It should not expose the underlying mapping because it would invalidate the purpose of read-only proxy. But there is a way to do this using comparison operator:

from types import MappingProxyType

orig = {1: 2}
proxy = MappingProxyType(orig)

class X:
    def __eq__(self, other):
        other[1] = 3

assert proxy[1] == 2
proxy == X()
assert proxy[1] == 3
assert orig[1] == 3

In particularly it allows to modify __dict__ of builtin types.