Oracle 11g: Part Copy of Production data to Development Environment
Your initial idea was not bad at all. What you can do is store the unwanted partitions with their local indexes in separate tablespaces. Use rman for the cloning but use the SKIP TABLESPACE option to not clone the tablespaces with the unwanted partitions. (assuming online backup)
After the clone, the skipped tablespaces have datafiles with status RECOVER.

see RMAN DUPLICATE DATABASE: Options

In the end you just drop the unwanted partitions. To be able to do that you first have to get rid of things like constraints and indexes that need to be re-created later on. This worked in 10gR2. Make sure that you don't drop the last partition of a table, in that case drop the table.

It is a bit of work but certainly possible. If the difference in Volume is huge or there are lots of copies, it might be worth spending some time for it.