I've looked into this and it's kinda possible, I don't have much control over it but for what you want it works fine, I'll add this next time I update it :}

I'm not sure what key/button or combination to use by default though, any ideas ? I will of course make it rebindable.

The middle mouse button is the default for this function in other games (eg. Minecraft, robocraft etc.)
But the key is busy, and now only a key modifier(ctrl) can help.
Alternatively such a key sequence can be added: P=palette; "open square bracket"=prev color; "closed square bracket"=next color; \=pick color.

ctrl and shift paint bigger areas when used with middle mouse, so that's not an option either.

I can't affect the vanilla binds and I'm not sure what you're talking about with that sequence.

On the one hand this button has to be convenient for use in construction.
On the other hand all this buttons are busy.
There is a solution: look at the cube and press button from 1 to 9 for 1-2 seconds and the block will occupy the appropriate cell on belt.

Holding down the button is good but will have a caveat... if there's nothing in the slot or you hold down the button after you already selected it on a tool (which deselects it) the block will be added to the first empty slot.

If the entire toolbar is full and you have no selected slots while you're holding, the first slot will be overwritten.

I could also prevent you from picking it if you have no slot selected but that will make it impossible to pick in empty slots.

Kinda fiddly but that's all I can do so far, unless you have other ideas to go around the fact that I can't pick what slot it puts the block in unless it's selected.

I have some questions:
1) Is it possible to know which slot is selected?
2) Whether it is possible to determine what is in the all other slots?
3) Whether it is possible to replace the item in a specific slot?

And generally let the better it will be uncomfortable, but somehow.

1. Yes.
2. Yes.
3. Not unless you have it selected.

As I said, the method only puts it in the selected slot OR in the first empty slot in the page OR if there are no empty slots it replaces the first slot in the page.

I'm not sure what you mean on that last sentence :P

> As I said, the method only puts it in the selected slot OR in the first empty slot in the page OR if there are no empty slots it replaces the first slot in the page.
So everything is logical now.
Thanks to the translator of the Evil's Corporation, without which I would have understood everything quite differently ;)

I support this feature; however I think the binding should be set to an unused key, instead of messing with the current game bindings.