Sorry, Chris. I've been thinking about this for a couple days, but I just don't have a solution for you. By the lack of other responses, it sounds like no one else on here does, either.

What Mac OS version are you running? Type 11 errors are strange...

This isn't an inherent problem with BootX, AFAIK. I've never not been able to run it. Do you have it installed so that BootX prompts you for an OS choice early in the Mac OS boot process? If so, what happens?

Which *nix are you going to run on the 5300? I've read that Linux has spotty support for the 5300...

Peace,
Drew
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Power to the PowerPC!