@Dark_Tundra
I think that would be possible. I haven't rummaged around too deeply in the sprite-directionality code, but as I recall it happens after buildings are loaded. Bars and windows and the like would also probably make sense in the same context. I'll take a look next time I'm down in those parts to see how much sense it makes.
Regarding the status of overlay-related things, progress is being made, though the time I have to put towards it is limited. I have constructions now using a similar system to the one for designations. Constructed tracks are handled by DF in a super strange way, but I'm pretty sure I got it all converted (reeeeeeeally long switch statement). What that means is that all constructions and designations will be handled in the same way as regular tiles, so if somebody wants to draw different sprites for them or make them a different color or whatever they can (the main motivation for doing this was to not have to duplicate the track-drawing code).
Next up is room-display (so you can see what's going on when you designate your dining hall or whatever), and then I need to figure out how multi-tile cursor stuff (like when you are plopping down constructions or buildings) happens. I'm a little concerned about the cursor because I honestly have no idea how it is being handled, but hopefully it isn't buried too too deep. There's more that needs doing after that, but those are the big show-stoppers that make the overlay unplayable in it's current state.
Keeping at it. Slowly.