Our coop apartment building is four floors, on irregular ground. The upshot
is, getting access to the windows makes painting and cleaning pretty costly.

Our painters/cleaners routinely use 60-foot ladders to get to the
upper-floor windows.

My question is this: Is it possible to fabricate some very sturdy hooks
which would fit over the sill of the windows, going over to the inside of
the building, to support a platform to do the painting and cleaning?

I realize this sounds dangerous, but it seems to me if the hooks and
platform could be very securely fastened, it would be made to be at least as
safe as those platforms dangling from ropes on high-rise buildings.