The parking brake surely works with FBW, we changed this just before release. And not only that, during the Beta, I had an email from somebody from FBW alerting they are using a custom parking brake variable instead of the standard, indicating the name of the variable we should use, and it's confirmed to be the correct one we already had.
Note that, I only tried the "Stable" version, is it possible the other versions use different parking brakes ? That seems unlikely, the FBW guy who contacted us would said something about it.