RMAN backup and restore of read only tablespaces

Due to the size (3 TB+, 650 tablespaces, 700 datafiles) of the database, and hardware throughput issues, it currently takes us over 16 hours to do a full compressed cold backup of our data warehouse. We are not archivelogging at the present time, but plan to start doing that soon (it was a space issue). We do have about 175 tablespaces (most of which have 1 datafile) that could be set to read only. What we have in mind (if possible) is to do a backup of of those tablespaces, then after it completes, set them to read only status. Then,