Could you not use some sort of CT arrangement to switch a contactor when the shower draws a load, removing the need to use the shower switch to turn the fan on and off?
Except you have the shower switch anyway, so when you pull it the fan switches on. And if you should ever want the fan on without the shower (e.g. a hot steamy bath) then you can do that, because the shower pull-switch is just an isolator, it doesn't make the shower come on.
<edit> Just seen your flow switch idea. That would be much more sensible
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