Did the lockout function on your Lefty forks? It does break.
Because otherwise, I'm pretty confused. The Lefty, and many other air forks, can lockout quite firmly. They do have a blowoff, of course.
Hell, even my originally-sold-in-2002 Super Fatty Ultra DLR80 headshok has a firm lockout. And is obviously air sprung.
...anyway, this has little to do with the Slate.