Specifying the entire path to the Mac OS ROM gave me different results:
0 > 5303ffff encode-int " AAPL,debug" property ok
0 > boot " cd:,\System Folder\Mac OS ROM:tbxi" load-size=2a8c4 adler32=639f2646
parsing <CHRP-BOOT>
evaluating <BOOT-SCRIPT>
After these lines printed out, the OS X grey apple logo screen appeared and the Mini basically just hung there doing nothing. No additional output appeared in the console.
I know the combo drive is good in the Mini because it can read discs without an issue. I think the disc I burned is good as well, because all the expected contents show up when the disc is in the drive in the mini. But my beige g3's DVD drive doesn't read the disc - maybe the drive is just too old now.
Other things of interest: the entire device tree
0 > dev / ok
0 > ls
ff899330: /cpus
ff8995c0: /PowerPC,G4@0
ff899a28: /l2-cache
ff89aa80: /chosen
ff89ac20: /memory@0
ff89ae20: /openprom
ff89af80: /client-services
ff89c1b0: /rom@ff800000
ff89c368: /boot-rom@fff00000
ff89c560: /macos
ff89c5f0: /options
ff89c680: /aliases
ff89e570: /packages
ff89e608: /deblocker
ff89ef58: /disk-label
ff89ffc0: /obp-tftp
ff8aabb0: /telnet
ff8ab460: /mac-parts
ff8aea88: /mac-files
ff8b1c20: /hfs-plus-files
ff8b7330: /fat-files
ff8bab08: /iso-9660-files
ff8bb918: /bootinfo-loader
ff8bd588: /xcoff-loader
ff8be058: /macho-loader
ff8c0ec0: /pe-loader
ff8c18c8: /elf-loader
ff8c3818: /usb-hid-class
ff8c6380: /usb-ms-class
ff8c90d0: /usb-audio-class
ff936bd0: /sbp2-disk
ff93a740: /ata-disk
ff93cd08: /atapi-disk
ff93f328: /bootpath-search
ff945960: /terminal-emulator
ff945a28: /firewire-disk-mode
ff95c4f8: /pseudo-hid
ff95c5b0: /keyboard
ff95cc30: /mouse
ff95d0d8: /eject-key
ff95d4d0: /pseudo-sound
ff95d660: /multiboot
ff9727b8: /diagnostics
ff972850: /xmodem
ff975218: /nvram@fff04000
ff9768f8: /uni-n@f8000000
ff976ab0: /hw-clock
ff978048: /i2c@f8001000
ff978c48: /cereal@1c0
ff979340: /pci@f0000000
ff9ce978: /uni-north-agp@b
ff9cec40: /ATY,RockHopper2Parent@10
ff9dd290: /ATY,RockHopper2_A@0
ff97a508: /pci@f2000000
ff97c8e0: /mac-io@17
ff97dd70: /interrupt-controller@40000
ff97dfe8: /gpio@50
ff97e5a8: /modem-reset@1d
ff97e700: /modem-power@1c
ff97e858: /extint-gpio1@9
ff97ea38: /programmer-switch@11
ff97efc0: /gpio5@6f
ff97f140: /gpio6@70
ff97f2c0: /extint-gpio15@67
ff97f4b0: /escc-legacy@12000
ff97f6d8: /ch-a@12004
ff97f888: /ch-b@12000
ff97fa38: /escc@13000
ff97fc70: /ch-a@13020
ff980880: /ch-b@13000
ff9813e8: /i2s@10000
ff9815d0: /i2s-a@10000
ff981988: /sound
ff9df600: /i2s-b@11000
ff981af8: /timer@15000
ff981ca0: /via-pmu@16000
ff985500: /pmu-i2c
ff9864f0: /temp-monitor@190
ff986af8: /local@0
ff986c90: /remote@1
ff987230: /rtc
ff987900: /power-mgt
ff9dfd10: /usb-power-mgt
ff987b40: /pmu-pwm-fans
ff988078: /fan
ff9882c0: /i2c@18000
ff988ee8: /cereal@1c0
ff9895e0: /ata-3@20000
ff98c300: /disk
ff99c0e0: /usb@1a
ff9a4a00: /usb@1b
ff9ad378: /usb@1b,1
ff9de320: /hub@1
ff9de5b8: /keyboard@1
ff9de9e0: /mouse@2
ff9b5cf0: /usb@1b,2
ff97b708: /pci@f4000000
ff9b6178: /ata-6@d
ff9b9360: /disk
ff9b99c8: /firewire@e
ff9c90a0: /ethernet@f
ff9dfe60: /ethernet-phy ok
And all the aliases for hardware: apparently the graphics card is called a 'RockHopper'??
0 > devalias
pci0 /pci@f0000000
agp /pci@f0000000
pci1 /pci@f2000000
pci2 /pci@f4000000
uni-n /uni-n
ui2c /uni-n/i2c
ui2c-serial /uni-n/i2c/cereal
keyboard /pseudo-hid/keyboard
mouse /pseudo-hid/mouse
sound /pseudo-sound
eject-key /pseudo-hid/eject-key
nvram /nvram
enet /pci@f4000000/ethernet
fw /pci@f4000000/firewire
cpu0 /cpus/@0
cpu1 /cpus/@1
pci /pci@f2000000
usb0 /pci@f2000000/usb@1b,1
usb1 /pci@f2000000/usb@1b
usb-1a /pci@f2000000/@18
usb-1b /pci@f2000000/@19
usb-1c /pci@f2000000/@1a
usb-2a /pci@f2000000/@1b
usb-2b /pci@f2000000/@1b,1
usb-2c /pci@f2000000/@1b,2
hd /pci@f4000000/ata-6@d/disk@0
cd /pci@f4000000/ata-6@d/disk@1
ide0 /pci@f2000000/mac-io@17/ata-3@20000/disk@0
ide1 /pci@f2000000/mac-io@17/ata-3@20000/disk@1
ultra0 /pci@f2000000/mac-io@17/ata-4@1f000/disk@0
ultra1 /pci@f2000000/mac-io@17/ata-4@1f000/disk@1
mac-io /pci@f2000000/mac-io@17
mpic /pci@f2000000/mac-io@17/interrupt-controller
scca /pci@f2000000/mac-io@17/escc/ch-a
sccb /pci@f2000000/mac-io@17/escc/ch-b
ki2c /pci@f2000000/mac-io@17/i2c
ki2c-serial /pci@f2000000/mac-io@17/i2c/cereal
via-pmu /pci@f2000000/mac-io@17/via-pmu
rtc /pci@f2000000/mac-io@17/via-pmu/rtc
pi2c /pci@f2000000/mac-io@17/via-pmu/pmu-i2c
wireless /pci@f2000000/mac-io@17/@30000
usb2 /pci@f2000000/usb@1a
fans /pci@f2000000/mac-io@17/via-pmu/pmu-pwm-fans
first-boot /pci@f4000000/ata-6@d/disk
second-boot /pci@f2000000/mac-io@17/ata-3@20000/disk
last-boot /pci@f4000000/ethernet
screen /pci@f0000000/ATY,RockHopper2Parent@10/ATY,RockHopper2_A@0