🚀 go-pugleaf

 The problem is not hard per se. If anyone was suitably motivated, that
sort of box could be developed. Instead of one output stage and a selector/
tuner, you would need a gang of outputs along with input and output traps and
summers. What you would end up with is a box with about 5X the circuitry.
(Assuming 5 pay channels. You also must be able to make assumptions about
where the pay channels are located in the channel block or be able to 
select which ones to decode).

 Since most people watch only one channel at once, or at most watch one and
tape another, most of the extra circuitry will be idle. I doubt that the 
difference in price would be justified - neither the cable company nor the
majority of the customers would be willing to pay that much extra for the
feature.