The way you originally drew it as a split flap is one thing, but then you were talking about separating it from the rest of the surface, and as I imagined what you were describing it sounded like an upside down Quicksilver single surface wing with a void behind the leading tube, which would make for horrible flow in an adverse pressure gradient.Why do you say it would separate? Just to clarify, the "dirty stuff" I am referring to would just be a couple of lever arms and some mounting tabs... i don't mean to imply that it would be completely devoid of decent airflow. Single-surface airfoils work well at low Reynolds numbers and at low speeds. In fact, they are quite efficient in that regime, no? Some of the low-Re eppler airfoils are actually profiled in a way that isn't too dissimilar to a single-surface airfoil. No?
Perhaps though, the LE should be closed off though to reduce drag, but the rest of the flap could still be a single sheet.
Single surface airfoils work quite well... for a narrow AOA range... unless they have something to create a large leading edge radius, in which case they're not really single surface any more.
Now if you could somehow make a sliding upper surface that telescopes aft and down to increase wing area and camber...