So it booted single user ok, but when I tried to run fsck -p it died.
Possibly interesting stuff:

root@$ fsck -p
/dev/rsd0a: file system is cleanxcall(cpu0,0xf02046e4): couldn't ping cpus: cpu1
; not checxcall(cpu0,0xf0204624): couldn't ping cpus: cpu1
king
xcall(cpu0,0xf020a480): couldn't ping cpus: cpu1
xcall(cpu0,0xf020a480): couldn't ping cpus: cpu1
xcall(cpu0,0xf0204624): couldn't ping cpus: cpu1
xcall(cpu0,0xf020a480): couldn't ping cpus: cpu1
xcall(cpu0,0xf020a480): couldn't ping cpus: cpu1
xcall(cpu0,0xf02046e4): couldn't ping cpus: cpu1
panic: lockmgr: no context
Stopped at      cpu_Debugger+0x8:       call            esigcode
db{1}> trace
cpu_Debugger(0xf025a228, 0xf02d5400, 0xf02d5400, 0x400000, 0xf02d4800, 0xf02adc00) at panic+0xfc
panic(0xf02d2ae4, 0x1, 0x0, 0x0, 0x0, 0xf5a46d00) at lockmgr+0x110
lockmgr(0xf02d2ae0, 0x7b000, 0x0, 0x2, 0xf5a32f28, 0x0) at uvm_fault+0x84
uvm_fault(0xf02d2ae0, 0x2a6, 0x7b000, 0xf5a46e30, 0x12ab0, 0x341cd) at mem_access_fault4m+0x264
mem_access_fault4m(0x0, 0x6, 0xf02d8918, 0x40000, 0xf0002000, 0x40) at memfault_sun4m+0xec
memfault_sun4m(0x7b000, 0x7f, 0x4, 0x0, 0xf02046e4, 0x3) at v8_srem_len+0x12208
0x1222c(0xf5a46fb0, 0xf0205d1c, 0xd00, 0x1e8000e7, 0x200, 0x3) at xcallintr+0x40
xcallintr(0xf5a34010, 0x0, 0xf022ae0c, 0x0, 0xffffffff, 0x800000) at softintr_common+0x150
softintr_common(0xf59fcd90, 0xf02d2dc0, 0x10, 0xf5a31220, 0xf02d4800, 0xf02adc00) at idle+0x8
db{1}> mach cpu
addr            cpuid   flags   curproc         fpproc
0xf5a02000      0       e000           0x0             0x0
0xf5a42000      1       9000           0x0             0x0
db{1}> mach cpu 0
using cpu 0
db{1}> trace
nmi_soft(0xf02d339c, 0x400042, 0x400040, 0x0, 0x3b419, 0x2995d7c0) at lsi64854_scsi_intr+0x4ec
lsi64854_scsi_intr(0x42, 0x0, 0xb00, 0xf0002000, 0x0, 0xf1d4f9d0) at _kernel_lock+0x14
_kernel_lock(0xab9e, 0x22060, 0xf0002000, 0x3, 0x200, 0x4) at intr_lock_kernel+0x4
intr_lock_kernel(0xf5a3b018, 0x0, 0xf022ae0c, 0xf02d7c00, 0xffffffff, 0x10) at sparc_interrupt44c+0x118
sparc_interrupt44c(0x0, 0x11, 0xf025b400, 0x0, 0x0, 0x10) at idle+0x8
db{1}> mach cpu 1
using cpu 1
db{1}> trace     
cpu_Debugger(0xf025a228, 0xf02d5400, 0xf02d5400, 0x400000, 0xf02d4800, 0xf02adc00) at panic+0xfc
panic(0xf02d2ae4, 0x1, 0x0, 0x0, 0x0, 0xf5a46d00) at lockmgr+0x110
lockmgr(0xf02d2ae0, 0x7b000, 0x0, 0x2, 0xf5a32f28, 0x0) at uvm_fault+0x84
uvm_fault(0xf02d2ae0, 0x2a6, 0x7b000, 0xf5a46e30, 0x12ab0, 0x341cd) at mem_access_fault4m+0x264
mem_access_fault4m(0x0, 0x6, 0xf02d8918, 0x40000, 0xf0002000, 0x40) at memfault_sun4m+0xec
memfault_sun4m(0x7b000, 0x7f, 0x4, 0x0, 0xf02046e4, 0x3) at v8_srem_len+0x12208
0x1222c(0xf5a46fb0, 0xf0205d1c, 0xd00, 0x1e8000e7, 0x200, 0x3) at xcallintr+0x40
xcallintr(0xf5a34010, 0x0, 0xf022ae0c, 0x0, 0xffffffff, 0x800000) at softintr_common+0x150
softintr_common(0xf59fcd90, 0xf02d2dc0, 0x10, 0xf5a31220, 0xf02d4800, 0xf02adc00) at idle+0x8
db{1}> ps
 PID             PPID       PGRP        UID S   FLAGS          COMMAND    WAIT
 44                42         42          0 3 0x84006         fsck_ffs  physio
 42                 7         42          0 3 0x84086             fsck    wait
 7                  1          7          0 3 0x84086               sh    wait
 6                  0          0          0 3 0xa0204         aiodoned aiodone
 5                  0          0          0 3 0xa0204          ioflush  syncer
 4                  0          0          0 3 0x20204           reaper  reaper
 3                  0          0          0 3 0xa0204       pagedaemon pgdaemo
 2                  0          0          0 3 0xa0204         scsibus0  sccomp
 1                  0          1          0 3 0x84084             init    wait
 0                 -1          0          0 3 0xa0204          swapper schedul
db{1}> 

I'll leave it this way for a little while if anyone has other things I
should check....

--sjg