Service call to be able to veto file close?

I was having a look at the bug https://www.riscosopen.org/tracker/tickets/385 where *Shut messes up USB networking, and I realised that it would also muck up MIDI too.

Problem is, there is no appropriate service call for files being closed. Sure, we can easily hook in to FileV and detect attempts to close file handle zero…only *Shut does not work like that. It makes an FSControl call to close all open files, where it will step through files closing them (with some behaviour for closing files within an image prior to the image itself), but by the time we get to where the business actually happens (FlushAndCloseStream in FileSys.FileSwitch.s.StreamBits), any failures here will be ignored and the file deallocated regardless – so trapping the attempt to close at this point would be useless.
I think it is here that FileSwitch ought to raise a service call so something that doesn’t want its files arbitrarily closed can veto that specific handle being messed with.

However, given the variety of ways to actually close a file, this service call should perhaps be raised prior to each individual file being closed. A sort of an “Okay to close file #123? Yay or nay?”.