Hi Paul,
I did some unsuccessful poking around in the manual and the MIPS code. I just stumbled across the following:
I can only presume that some machines have bootloaders that clear ERL before the bootstrap code is invoked.
Judging from this sentence in the Status.ERL description of the manual: "The operation of the processor is UNDEFINED if the ERL bit is set while the processor is executing instructions from kuseg."
I say that this bit should be cleared. The two 256MB regions also point to the intention that Status.ERL is clear and thus there is no unchached+unmapped useg.
I'm sorry to not have more helpful comments.
Cheers Philipp