What a great thread, thanks @petewill for all of the info. I can't wait to give this a try.
After reading through all of this, am I correct in assuming that using the slider control (or programming a controller to open the shade to a certain %) has not been achieved?
I was thinking about this a bit, and thought about perhaps using a few cheap magnetic door sensors along the edge of the window at different positions. You could then attach a small neodymium magnet to the back of the bottom edge of the shade that passes by the door sensors. Each of those door sensors could report a value back to the controller, indicating its position (ie 0,25,50,75,100), and if the MySensors node receives a slider value from the controller, you could round to the closest value, and raise/lower the shade until that value is achieved.
I'm a complete MySensors/HA/Vera noob, so please excuse my ignorance if this is a ridiculous solution to a simple issue.