I don't think it would work.
For one thing, milled receiver AKs do not have seperate barrel trunnions. If they did, you could make the aluminum reciever with a steel barrel trunnion and not have to worry about blowing the gun up. But if you're thinking of the bolt locking into the aluminum receiver and the barrel being pinned or screwed into it, you're asking for trouble. Aluminum locking lugs the same size as the steel ones on an AK would be a HUGE no-no.
Secondly, ARs get away with the bolt carrier riding inside an aluminum receiver because they have much more surface contact between the bolt carrier and the receiver. This greatly increases the wear life of the aluminum reciever. If a steel bolt carrier were riding on small AK rails made of aluminum, they wouldn't last very long.
Thirdly, the AR design is less stressful on the carrier and reciever do to the spring buffer in the stock and the light weight of it's moving parts. An AK built with an aluminum receiver, even a milled one, would likely overstress the reciever.
Lastly, a milled aluminum receiver would not likely be as rigid as a stamped steel one. The wall thickness of a milled receiver isn't much greater than a stamped one once the lightening cuts are done. If you 'downgrade' the milled receiver to a softer/weaker metal, you lose the benefits the milled receiver had in the first place.
As others have said, I see very little difference in the weight of a milled AK and a stamped AK. The difference between a loaded 7.62 rifle and a 5.45 rifle is much greater. Stamped AKs were brought around because they could be made more quickly and cheaply than a milled receiver, not because they were lighter.