It's not an actual crash I want to debug. I want to use ddb to track
   down some inconsistent behavior. (See the PR link for more information).
    Hmm... How huge are we talking about here? I have constraints, since
   I'm booting from a memory disk (loaded from an USB memory stick).


ddb doesn't use much debugging info.  it won't give you
sources or line info like gdb does.  you don't want to
boot the netbsd.gdb.

perhaps you want to use kgdb with a serial port?  or
simply plug numbers into gdb while driving them from ddb.
i do the latter a lot -- inspecting the pc addresses etc
from gdb that i found in ddb.