Well, I got closer. From the Plan 9 Install directions:
IDE/ATAPI drives in Plan 9 are named like sdD0, where the capital letter is C for the primary IDE interface and D for the second. The digit is 0 for the master and 1 for the slave. So if your CD-ROM is the primary slave, use sdC1:
So I got it to boot into the Installer GUI, but it Paniced:
panic: corrupt tail magic0
pool Main block f0b77718
bla bla bla......