Can an aircraft have a lifting fuselage that only generates lift at certain speeds?
I think you are looking at this backwards...  (at least for a utilitarian passenger type aircraft)
First, such a design is most certainly possible, but it would create more problems than it would solve.  Wings don't need extra help producing lift at cruise speeds, that is when they are most efficient.  Instead, wings can use a little extra help at slower speeds, which is why they are typically equipped with lift enhancing features like flaps and slats.
Second, producing lift creates induced drag.  You want the fuselage to be as streamlined as possible to produce the least amount of drag at higher cruise speeds, not more.  And if your lifting fuselage is fully stalled at all other phases of flight it will be producing MASSIVE amounts of drag.
This full-stall drag penalty would create significant performance problems in all other phases of flight that would likely cancel out any small efficiencies at cruise speed, such as redesigned wings, more powerful engines, etc.  I don't see anything to be gained by such an idea.
And finally, what makes you say that we are 20 years away from a blended wing body design being able to fly?  There have been all kinds of designs built and flown in the last century, I don't think there are any technological hurdles to this type of aircraft.