If your database is read-only then your log file is not going to be used much and you're not going to need point-in-time recovery, so there's no harm in having data and log on the same drive.  If you lose the drive, you just restore from backup.  It's still a good idea to have tempdb on a separate drive, though.  If your server is a VM, speak to your SAN administrator about whether there's any performance benefit of having separate drives.

John