Kernel hang r247079 mps/vfs/zfs?

I was testing a patch on r246300 or so, and wanted to see if it would apply
cleanly to a newer copy of HEAD.

Well it did, except I had a hang at boot, shortly after ZFS version and the
last scsi devices appear.
This easily could have been related to the patch I was testing, so I wiped
/usr/src/ and /usr/obj (after booting kernel.old) and rebuilt world and
kernel cleanly.

I assumed that would resolve the issue, but it did not.

The hang happens right after zfs is announced, and the last da devices
(some of which are usb) are reported. It comes before the noisy output of
mps.

Hang is complete, and single user or verbose don't yield much.

I'm having trouble exfiltrating a dmesg from it, but it may be unrelated to
the userland issues reported earlier, as single user does not resolve it
for me.
The svn up was at 11:20 pacific (GMT +8).

Anyone else seeing similar issues?

Hardware is an LSI mps device, "9210" crossflashed m1015. Pool is a zfs
mirror. Works fine booting from r246300 kernel.
Motherboard is an AMD Tyan.
Pulling USB headers off the board didn't resolve it.

Matt