Been quite a few months since I worked on this. Reworked the input library, which itself is a W.I.P like the rest, and designed a new background. Pictured is a test of the window and input libraries.
I made a cool-looking 3D gauge.
Pro tip: you can use a BG layer as a second general-purpose graphics layer that's at a different depth. Just fill the layer with tiles arranged the same way they are in the BG graphics page (#5 by default), set that page as the manipulation page (GPAGE 0,5), GCLS, and draw to that page like normal.