>So after this, what's the entry in METALOG for postdrop look like?

As I mentioned the METALOG entry is fine.

>  | I suspect the damage is happening in maketars, but haven't had time to
>  | look into it.
>
>I can't see how with an unprived build; maketars is unconditionally
>invoked with '-N ${NETBSDSRCDIR}/etc'.   :/

Sure, but I'll bet there's a step it does where it does not pass on
$etcdir.  Otherwise I can't account for the fact that all the input
looks correct, but the set list (or whatever its called) that gets
generated during maketars has the wrong group info - and its a case of
gid mapping to wrong name.

--sjg