I don't know if this is helpful to anyone.  I believe the behaviour was 
similar to what Michael
Hitch had described when trying to do a reboot 10c or reboot 0 from the 
debugger.  So when an MP
system crashes is it the luck of the draw as to whether you'll get a 
debugger or not?  Does CPU 0
always get the console?  So when CPU 0 hangs, CPU 1 drops to a debugger 
but you can not
access it because CPU 0 has the console?

cpu_Debugger() at netbsd:cpu_Debugger+0x4
comintr() at netbsd:comintr+0x798
alpha_shared_intr_dispatch() at netbsd:alpha_shared_intr_dispatch+0x6c
sio_iointr() at netbsd:sio_iointr+0x4c
interrupt() at netbsd:interrupt+0x304
XentInt() at netbsd:XentInt+0x1c
--- interrupt (from ipl 0) ---
idle() at netbsd:idle+0x70
idle() at netbsd:idle+0x54
--- root of call graph ---
db{0}> ps
  PID           PPID     PGRP        UID S   FLAGS LWPS          COMMAND 
    WAIT
  16077        16025    16077          0 2    0x10    1             cron 
  vnlock
  15958        11223    15958          0 2    0x10    1             cron 
  vnlock
  11092          399      399          0 2  0x2000    1            inetd 
biowait
  16025          467      467          0 2       0    1             cron 
  ppwait
  11223          467      467          0 2       0    1             cron 
  ppwait
  11755            1    12622      32767 2     0x2    1         upclient 
  vnlock
  656              1      656          0 2  0x4002    1            getty 
  vnlock
  424              1      424          0 2       0    1        rpc.statd 
  select
  76               1       76          0 2  0x4002    1              ksh 
  vnlock
  467              1      467          0 2       0    1             cron 
  vnlock
  399              1      399          0 2       0    1            inetd 
  kqread
  408              1      408          0 2       0    1            timed 
    poll
  105              1      105          0 2       0    1        rpc.lockd 
  select
  101              1      305          0 2       0    1             nfsd 
    nfsd
  100              1      305          0 2       0    1             nfsd 
    nfsd
  99               1      305          0 2       0    1             nfsd 
    nfsd
  98               1      305          0 2       0    1             nfsd 
    nfsd
  97               1      305          0 2       0    1             nfsd 
    nfsd
  96               1      305          0 2       0    1             nfsd 
    nfsd
  315              1      305          0 2       0    1             nfsd 
    nfsd
  311              1      305          0 2       0    1             nfsd 
    nfsd
  309              1      305          0 2       0    1             nfsd 
biowait
  307              1      305          0 2       0    1             nfsd 
  vnlock
  308              1      305          0 2       0    1             nfsd 
    nfsd
  302              1      305          0 2       0    1             nfsd 
    nfsd
  312              1      305          0 2       0    1             nfsd 
    nfsd
  318              1      305          0 2       0    1             nfsd 
    nfsd
  304              1      305          0 2       0    1             nfsd 
    nfsd
  313              1      305          0 2       0    1             nfsd 
    nfsd
  285              1      305          0 2       0    1             nfsd 
    nfsd
  242              1      305          0 2       0    1             nfsd 
    nfsd
  301              1      301          0 2       0    1           mountd 
  select
  239              1      239          0 2       0    1           ypbind 
  vnlock
  182              1      182          0 2       0    1          rpcbind 
  vnlock
  188              1      188          0 2       0    1          syslogd 
biowait
  11               0        0          0 2 0x20200    1         aiodoned 
aiodone
  10               0        0          0 2 0x20200    1          ioflush 
biowait
  9                0        0          0 2 0x20200    1       pagedaemon 
pgdaemo
  8                0        0          0 2 0x20200    1        atapibus0 
  sccomp
  7                0        0          0 2 0x20200    1          atabus1 
   atath
  6                0        0          0 2 0x20200    1          atabus0 
   atath
  5                0        0          0 2 0x20200    1         scsibus1 
  sccomp
  4                0        0          0 2 0x20200    1     isp0:fc_thrd 
fcclnup
  3                0        0          0 2 0x20200    1         scsibus0 
  sccomp
  2                0        0          0 2 0x20200    1        cryptoret 
crypto_
  1                0        1          0 2  0x4000    1             init 
    wait
  0               -1        0          0 2 0x20200    1          swapper 
schedul

Then trying to do a dump:

panic: alpha_send_ipi: CPU 1 not running
panic: alpha_send_ipi: CPU 1 not running
panic: alpha_send_ipi: CPU 1 not running
panic: alpha_send_ipi: CPU 1 not running
panic: alpha_send_ipi: CPU 1 not running