To save someone all the pain I went through to get a PPC Mac to boot from an enclosure with the Oxford 934 chipset
* Enable22TBCapacity needs to be disabled (zero) in the configuration
so this is only going to work for drives less that 2.2 TB, an APM (Apple Partition Map) limitation
* the firmware should not be more recent than 1.20.0101 (Aug 2009)
this one almost got me
With the above, an APM formatted hard drive should be able to boot on a PPC Mac (did for me)
This process needs to be done from a PC as it needs the Oxford uploader 3.06 or above, and I could not locate such a version for the Mac (version 3.00 is not recent enough)
If one wants to access more than 2.2 TB, set Enable22TBCapacity to 1 and use GUID formatting; but one will then not be able to boot from a PPC Mac
Disclaimer: if not done right one can end up with a bricked enclosure, so proceed at your own risk. If one does brick the enclosure, working through firewire on a PC may help rescue the unit (been there, done that 
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Some history
* I updated my MacAlly enclosure firmware from 1.1 to 1.2 (http://www.macally.com/EN/techsupport/drivers/PHRS100SUA_GS350SUA_GS350SUABfw.zip) and lost the ability to PPC boot; the problem in this case was not the firmware but the configuration file moving Enable22TBCapacity from 0 to 1; I moved this setting back while retaining the more recent firmware, and all was well again.
* I recently got a G-Drive enclosure with firmware version 1.20.0205 (Dec 2010) and here I needed to downgrade to 1.20.0101 (Aug 2009); the Enable22TBCapacity setting was not an issue despite the fact that the enclosure originally held a 3 TB drive.
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Sometimes I wonder if it would not be less trouble to just move to an Intel machine, but then again, life is not always about taking the easy way; Steve Jobs was not the first to recognize that ‘the journey is the reward’, for in life the destination truly sucks.