From reading these two forum posts:
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I think you might be able to use either a CDD or a screen with the "key" keyword detecting mouse events.
So in a Screen, use "key" to detect mousedown, and mouseup, triggering a variable change "pressed=True".
Then in short repeating timer that only runs if the variable is true, call a function.
In your funciton you need to check the mouse position is within your click target, and while it is, keep doing whatever you need the "button pressed" state to do.
When the mouseup event is detected, reset the variable which stops the timer.
You might need some interlocks varaibles or something to handle the mouse position going outside the target area and going back in again.
It's not going to be simple though - Renpy is not primarily designed to be a good platform for realtime event handling (ask me how I know
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