I have a somewhat similar script that works in the arrange view and lets you paint in items:
https://streamable.com/5r2op (Windows only)
Disclaimer: It has some issues and is unreliable at times so I don't have a thread for it. Use at your own risk.
To install add this to your reapack.
Then search and install BirdBird - Item Sequencer
You also need to have SWS and js_ReaScriptAPI installed.
Some info:
It paints the first items on the track
Left dragging paints in items
Right dragging erases items
Holding A and clicking on items lets you drag up and down to change their volume
Holding S and clicking on items lets you drag left and right to offset them
Holding Q deletes the grid area you are hovering over no questions asked
Known issues:
Doesn't work on OSX
Script leaves lingering rectangles on the screen if it crashes *
Using it too fast misses grid divisions *
Does not work with swinged grid divisions *
The ones I marked with * have solutions and I may get around to fixing them in the future, but currently don't have enough time to spare for it.