Gee, this worked:

I have a read only tablespace READ_ONLY and I have backup this up. Then I have in RMAN: configure exclude for tablespace READ_ONLY. Thus my backups do not contain the READ_ONLY tablespace.

Then: drop tablespace READ_ONLY including contents and datafiles;

How to recover:

1. run{
set until time "to_date('11.12.2003:12:58:01','dd.mm.yyyy:hh24:mi:ss')";
restore database;
recover database;
}

2. alter database open resetlogs;

3. alter tablespace READ_ONLY offline;

4.

run{
SET NEWNAME FOR datafile 3 to '/home4/user8/ORADATA/u03/read_only.dbf';
restore datafile 3;
switch datafile 3;
}

5. recover tablespace READ_ONLY;

6. alter tablespace READ_ONLY online;

All is fine, the data is in the databse.