Track changes to readonly

I wonder how it is possible to react to changes to a buffer's readonly flag both,
- after starting vi, e.g. with gvim -c "set ro" (not gvim -R), and
- when changing the flag via the command line.
So far I haven't found an event covering these cases.
I want to change the background ("highlight Normal") dynamically based on the buffer's state.

Any ideas?

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