It appears to work (but doesn't have much effect without an rs6000 
unwinder).

One question (and to follow up my earlier post) is there a better way of 
doing this:

+  if (frame_pc_unwind (next_frame) > 0x7f000000)
+    /* Assume anything that is vaguely on the stack is a signal
+       trampoline.  */
+    return &ppcnbsd_sigtramp_unwind;

ok?, eventually for 6.1?

Andrew