Is there a gadget that will let me slide my seat forward more than my post allows?
No. Based on your question and your comments, there is not a product that does what you are seeking (get your hips over the bottom bracket). The reason being that no one rides that far forward.1 The old style Profile is probably your best bet for getting forward:



There are designs with a seat over the bottom bracket, even some that are made super-adjustable in that regard:



Even at that far forward, the hips are behind the bottom bracket:



With that in mind, a reason that most people don't ride that far forward is because moving yourself that far forward screws with the bicycle handling and puts you too far over the front wheel. Note that bikes that enable the most aero positions, like those pictured above, achieve the effect not by moving the saddle forward, but by moving the bottom bracket back.

Now, with all that lecturing done, I would say that if you are dedicated to this experiment, you can get close with the profile seatpost mentioned and an ISM saddle. The ISM is an anatomical design with two points and a gap between. A lot of riders slide forward on that saddle and hook their sitbones on the two points. This is opposed to traditional time trial saddles which just had more padding on the nose meaning that sliding forward just jams that thing into you perineum. Even better is if you can start with a bike with a steeper seat tube.

1That is a bit of hyperbole, perhaps someone rides that far forward.