FYI. RMAN addresses tablespace recovery without a baseline backup.

Here's how. Eventually, asked Tom about it. Embarassingly for me, he pointed me to the user guide. :(
Recovery is identical to how it was performed with traditional Oracle hot backups.


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Recovering a Lost Datafile Without a Backup: Example
In this scenario, the following sequence of events occurs:

You make a whole database backup of your ARCHIVELOG mode database.
You create a tablespace containing a single datafile called ?/oradata/trgt/history01.dbf.
You populate the newly created datafile with data.
You archive all the active online redo logs.
Someone accidentally deletes ?/oradata/trgt/history01.dbf from the operating system before you have a chance to back it up.
Are you prevented from recovering the data in the lost datafile because you have no backup of the file? No. You can recover the data by creating a new datafile with the same filename as the lost datafile, then run the RECOVER command to apply the redo for this file.

For example, start RMAN, connect to the target database, and then run the following statements at the RMAN prompt:

# take the missing datafile offline
# note that SQL statement is bounded by double quotes, but the datafile name has two
# individual single quotes both before and after it
SQL "ALTER DATABASE DATAFILE
'' ?/oradata/trgt/history01.dbf '' OFFLINE";
# create a new datafile with the same name as the missing datafile
SQL "ALTER DATABASE CREATE DATAFILE
'' ?/oradata/trgt/history01.dbf '' ";
# recover the newly created datafile
RECOVER DATAFILE '?/oradata/trgt/history01.dbf';
# bring the recovered datafile back online
SQL "ALTER DATABASE DATAFILE
'' ?/oradata/trgt/history01.dbf '' ONLINE";