Look look look what I found! The text of "Double Falshood; or, The Distrest Lovers". The Theobald play in question, which could indeed be the lost Cardenio (or at least a revised and adapted version). Whether it is or isn't, I certainly wasn't going to pass up the chance to read it!
I don't know who this "jwkennedy" person is who did the transcribing work, but he's my new BFF :). Thanks!