You can use the CD directly from a normal mountlist without having to define it in the harddisk section.
Firstly, enable UAE scsi.device in the Misc tab of UAE prefs.
Then, make sure you have the relevant dosdriver (CD0) from CacheCDFS supplied with 3.9. You need to edit this to use uaescsi.device. Mine is Unit 0 but you can try other values if this doesn`t work.
You may also need to perform a 'diskchange cd0' to make workbench see the disk.
Hope this helps,
Duncan.