It doesn't seem to be possible to override the role of an application (or any other property set by the application) with environment variables. Using PULSE_PROP_OVERRIDE doesn't work, because it's applied on the context proplist only. The context proplist is copied to the stream proplist, but if the application sets any properties itself, those will of course override the context properties.
Maybe it would be a good idea to add new environment variables for setting stream properties: PULSE_STREAM_PROP, PULSE_STREAM_PROP_OVERRIDE etc.
I changed my mind about the preferable fix: I think it would be better to just make sure that PULSE_PROP_OVERRIDE overrides also the stream properties.
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