RMAN restore to lower disk space
One of the first things Oracle does at restore time is pre-create the data files to the defined size. So you do need a way to store the full size of the files. A compressed filesystem could be used for that and with a little luck it works. It just does not solve the problem.

What you do want is to re organize the source database. The quickest way is to prepare a new database with correct sized tablespaces and datafiles. Next use datapump to transport the schemas to the new database. If needed, this can even be done online by using smart tools like dbvisit replicate (or goldengate if money is of no importance). Trash the old database when the two are in sync.