It seems we can reuse an existing MDS schema when running the Repository Creation Utility (RCU). Previously I’d always created a new one for each installation but I guess I should have paid more attention to the Oracle documentation…
If your organization has an MDS schema installed in a supported database, you do not need to install the MDS schema again with RCU. You can use the existing MDS schema for your Oracle Business Intelligence installation. For this option, deselect the Metadata Services schema in the Select Components screen of RCU. Then, when you install Oracle Business Intelligence, specify the details for the existing MDS schema in the MDS Schema screen.
You always need to create one BIPLATORM schema for each instance you install – it’s only the MDS schema that can be shared. More info here.
I hope this helps you keep things neat and tidy – should save a few Mb in the database too!