I still haven't got the dratted thing to give me console access. But I have managed to install the beast in such a way that I can finish the install and log in.
The first step is to ensure that you have ipmitool installed. If you have Solaris 10, you may have it, and if so you need to patch it (119764 or 119765 according to your system). Then I can tell it to do a pxe boot next time around:
ipmitool -U root -I lanplus -H 126.96.36.199 chassis bootdev pxe
And I can power cycle it and it will do a jumpstart install.
Then in my finish script I have
sysidconfig -b /a -r /usr/openwin/bin/kdmconfig
which stops it asking me whether I want Xorg or Xsun at boot (questions which I can't answer because I have as yet found no way of typing back at it).
And I have a working system I can access using ssh. With luck I can get the installed system to play nice with the console, which is going to be essential when we move this box into production use.