My external ROM is actually disabled throughout this (didn't want to introduce additional issues), so address zero, plus all the normal Exxxxx space is being decoded and serviced by the motherboard.
I'm back to my old clock switching code so the clock is only ever at the motherboard's speed when asserting XAS. Quite, therefore, why there's a problem around reset, I've not worked out -- but holding off the 'allow acceleration' flag until later in the boot process seems to work.
In effect, it's not doing any clock switching until the OS tries to make the jump to 16MHz (I'm decoding the call to that bus control register).
Odd, but not sure I'm going to go chasing why just yet.
BW