...now appears to be recognising my mouse at boot-up.  I haven't yet been
able to test it in X, as X doesn't appear to like the kernel that I've
used to try out my solution.

I don't know if what I did was right - it's almost completely empirical.
I altered the IOMD_MSDATA and IOMD_MSCR in iomd/iomdreg.h by dividing them
by four.  (The ARM documentation shows that the offset in bytes is &A8 and
&AC, which is what these were defined as, whilst all the other registers
were defined as the offset in words.)

Hope this is of use to someone,

David Forbes.