I look forward to seeing the working example. Hopefully it's clear already, but: I don't think anybody is yet claiming the descriptor approach is the "correct" or "best" answer for you. But it will help a lot to demo what you want, and it also allows you to use your first choice operator, which is =.
 
Problem remains that you cannot pass hdlns.x/y around, x and y are
really the things you want to pass around. You can pass hdlns in this
case, but the receiving object has to figure out which signal (x or y)
to use.

If you can pass x around, you can certainly pass hdlns.x around, or something shorter if you prefer-- ns.x, perhaps.

The problem of: "the receiving object has to figure out which signal (x or y)
to use" seems easily addressed by creating a Signal class that knows how to return the Right Thing when doing math:

from numbers import Real  # or whatever number abc is appropriate
class Signal(Real):
    # override all mathematical operations

And modify the __set__ method of the SignalBehavior descriptor so it stores a Signal:

class SignalBehavior:
    ...
    def __set__(self,inst,value):
        self.inst_dict[inst] = value if isinstance(value,Signal) else Signal(value)

Now any math operation you care to do can result in whatever you wish it to be:

>>> ns = HDL()
>>> ns.x = 1
>>> ns.y = ns.x / 2  
>>> ns.z = 8 / ns.y