The concept has been studied by NASA in the early 60s:
Tecnical report NASA TM-X-893, dated Oct. 1963:
"Longitudinal stability and aerodynamic characteristics of a reentry vehicle configuration having an extendable leading edge at a Mach number of 10.03 and angles of attack from 50 deg to 90 deg"
That's interesting. But there were also other heat shield concepts being floated around even in the late 1950s. I saw a document at the Smithsonian a few years back which I believe was an AVCO proposal for an inflatable heat shield. I forget the name, but it was essentially a very high drag, low mass system. Sort of like a big umbrella that would inflate in front of the craft. I suspect that materials would have been a major problem, then much more so than now.
That's it. I knew that it had an odd name, and "drag brake" is it. However, the proposal I saw predated Dyna Soar. It might even have predated Mercury--maybe 1956 or 57. I was struck by the fact that it was an alternative capsule reentry technology than what they went with for Mercury.
there's a NASA history interview (I think it was just a transscribed tape) with Max Faget and some others and they talk about, among other things, the diversity of pre-Mercury re-entry ideas. One was a steel parachute (IIRC an idea by some professor or some company) that Faget said he showed that would melt. I can't find it anymore. IIRC he also tested the oscillations of re-entry shapes in a water tank and iterated the shapes that way, and used jet engines to test heat shields.