Hmm.
I was about to suggest picking up
this card, when I read the following in the description:
Internal Drives Supported Supports any ATA/IDE fixed media hard drive including ATA/33,ATA/66, ATA/66Removable media/ATAPI devices supported under OS X
So it looks like you would not be able to use that for a DVD-R drive under OS 9. That is something to watch out for - make sure the card supports ATAPI (removable media) drives. Same goes for adapters from Firewire or SCSI.
SCSI DVD writers are very hard to find, so I wouldn't bother going with a SCSI card myself.
I think the best bet for OS 9 would be a Firewire card and a Firewire-ATAPI adapter. You could pick up a cheap external Firewire CD drive and replace the drive with a DVD-R, or remove the converter card from the box and mount a drive inside the Mac. Keeping it external means you can use it on other machines of course.
On the other hand, an external SCSI box and a
SCSI-ATAPI converter (US$20) would give you a CD/DVD drive you can use on other old Macs, without needing a FW card for every machine. That would include Nubus /PDS Macs and Powerbooks.
I guess it comes down to your range of machines, and which ones you'd like to have a drive for.