I just implemented the -a and -r options for the attr command, so you can add some devices to further rooms with one command:
attr -a Bedroom.* room ,Bedroom
To remove them use:
attr -r Bedroom.* room ,Bedroom
The comma (,) at the beginning is necessary, as rooms are separated with comma.
In all other cases a space will be inserted automatically.
The -a is not necessary if the device has no rooms assigned yet.