This is something I have been thinking about too! I really like a similar feature in Blender, where holding Ctrl toggles “eraser”/“invert”, and releasing it turns the brush into “draw”/“normal” mode again!
I would love to have that in Krita! There are many times I forget about toggling the eraser!
There have been this topic on improving the existing one to avoid forgetting about toggling Having a visual change when in eraser mode (community poll) So the future looks brighter for the Eraser mode!
But I would love to have a press and hold like the one you describe! The mix of “hold to activate - release to deactivate eraser”, and the visual change would be fantastic, a dream come true! 
I don’t think this is possible in Krita right now, I hope I am wrong and that someone has a solution.
This would be a fantastic Feature Request! 