You might want to try asking in the Z4 discussion section?
I've come across front and rear bumper but this is slightly different. Don't think it's been discussed often but I'm sure it's been done and someone will be able to help you out. Are you trying to remove right now or researching how to remove? I'd probably start with removing the arch lining and seeing what's there. I'm sure there will be something connecting from possibly roof area? So might need to remove the soft top!