Maybe you mean COMPAT_DARWIN?

man options
...

options COMPAT_DARWIN
On those architectures that support it,this enables binary compatibility
with Darwin applications built for the same architecture.  This feature
is highly experimental, it requires COMPAT_MACH and EXEC_MACHO and it is
currently limited to i386 and powerpc ports of NetBSD.

The last status report I read said the OS X window server would display, 
but he needs help to go further enough for it to be useful.

http://mail-index.netbsd.org/port-macppc/2004/08/14/0005.html


Shane Ambler wrote:
> New version of the same thing.
> 
> Apple bought NeXT - Mac OS X is really just a new version of NeXT OS so
> there may be some more recent changes if development came from NeXT OS days
> then there may be some extra changes to be done.
> 
> On 23/4/2006 13:45, "der Mouse" <mouse@Rodents.Montreal.QC.CA> wrote:
> 
>>> You may find that the aim of COMPAT_MACH is to get the mac gui to run
>>> on top of netBSD but I think it would be a bit early to expect this
>>> to run at this stage.
>> I thought it was actually designed more around NeXT software (though of
>> course I didn't do it so I don't know), with OS X compat a bonus.