Another approach is to abandon the idea of HDL namespace parent class, and just create a HDL (or hdl) decorator:
@hdl # decorator adds a custom __setattr__ class C: ...
The decorator would modify the __setatrr__ method of the class so that members of the class have signal behavior. You could even write the decorator so that you specify which attributes have signal behavior, and the remainders don't:
@hdl(list('xyz')) class C: def __init__(self, **kwargs): for k,v in kwargs.items(): setattr(self, k, v)
c = C(x=1, a=2) # x is a signal, a is not