CURRENT kernel crashes on boot with BTX error

I've been trying to build an 8-CURRENT kernel on my 7.0-RELEASE system in 
order to test out some patches that Pyun YongHyeon has prepared. 
Unfortunately, I cannot get a GENERIC+DDB kernel to boot on my 
machine.

This is an Intel SR1200 Pentium 3-class system - there is a dmesg from 
7.0-RELEASE in the first part of 
http://www.freebsd.org/cgi/query-pr.cgi?prp=125769-1-txt&n=/patch.txt

I'm not really sure if the process I'm using is supported, so I will 
detail that first:

(CSup to Tuesday 21 Oct 2008 1100Z sources from cvsup6.au.freebsd.org)
/usr/src-8# make buildworld && make buildkernel KERNCONF=GENERIC-DEBUG
/usr/src-8# make installkernel KERNCONF=GENERIC-DEBUG INSTKERNNAME=gd-8
/usr/src-8# nextboot -o "-D -h -s" -k gd-8

The kernel loads, the various modules I have specified in loader.conf are 
loaded, and the loader screen appears. However, when I select "boot", the 
expected line with the kernel version does not appear: instead, the 
spinner briefly spins, switches to the bold colour, and then crashes with 
a BTX error:

int=0000000d  err=00000000  efl=00010a13  eip=00000430
eax=ffffffb4  ebx=00006c47  ecx=0000000a  edx=00000080
esi=00000001  edi=ffff9414  ebp=00000000  esp=0008f8b4
cs=002b  ds=0033  es=002b    fs=0033  gs=0033  ss=0033
cs:eip=6c 7f 94 48 00 00 00 00-0f af c1 47 00 00 00 00
       00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00
ss:esp=2b 00 00 00 33 00 47 91-00 fc 03 00 5f ad 08 04
       00 00 00 00 3f 00 00 00-00 00 00 00 c4 0f 06 00
BTX halted

(Copied by hand because I can't convince the loader to output BTX errors 
to serial console.)

Other modules being loaded on boot are geom_mirror, geom_vinum, ipmi and 
smbus. I tried with just geom_mirror and the same thing happens; loading 
without ACPI also provokes the same error.

So, my issues:

1. Is this an acceptable method of testing a CURRENT kernel? I also tried 
building a new world and kernel with DESTDIR set to another partition but 
had similar problems. Unforunately this machine does not have a reliable 
CD drive so I can't use a snapshot CD to install.

2. Any hints on where to go with the BTX error? I have compiled DDB into 
this config, but haven't convinced the kernel to break into the debugger. 
What little I've found on the web about BTX debugging suggests that it's 
something of a black art.

Thanks,

David Adam
zanchey_at_ucc.gu.uwa.edu.au