Great work! This is easily the mod of the year, especially if you make a few improvements:

1) The ability to turn it on/off with a hotkey. This mod does not work if you want to quickly lay down large belt lanes (since then you want to lay them in the same direction both when running up and down). I'd suggest making it a per-player option if the hotkey should be hold or toggle.
2) It would be great if I could override existing belts the same way I lay new ones. I guess not all users would want it, so again an option.

The on/off toggle is already on the wishlist.

For laying multiple lanes at once, I still prefer/advice using bots to lay them, by having a blueprint of 2wide or 4wide beltlanes, and pasting them. I did see another belt mod on this site that lets you lay multiple lanes at once, but have no experience with it.

Not sure what you mean with 2. One thing that's high on my list though, is that user actions should always have precedence over auto mod actions. Meaning that I don't want a mod to guess too much, and thereby making your game more about fighting the auto-lay system, opposed to a mod system that actually tries to help you. An overlayed belt is something what I see as: Mod did something, user wants it differently. That's why an overlayed belt will not be changes by the mod.

Yeah I agree that bots are better for multiple lanes, it was just an example. About 2... I mean if I click on an existing up-belt and drag to the left, I want it to change direction to the left. I feel that this would be more in line with the purpose of this mod. I don't want to hit R at all when placing belts, unless I've toggled this mod off. I've used this for two days now, and I find it hard to get into the flow of "drawing" belts when I cant use it for "editing" old belts.

I'll keep the drawing over old belts in memory, can't say more about it now