Greetings.

I've added DESTDIR support to pkg_add(1) in OpenBSD
some time ago.

It's triggerred by the -B option (mnemonics: base directory)
It can also be set in the environment as PKG_DESTDIR.

At the time, -D was used in pkg_delete to mean `do not execute
deinstallation scripts', so I chose an unused letter, instead of
confusing the user more.

(since DESTDIR makes sense both for pkg_add and pkg_delete).

I explicitly did NOT choose the -C of FreeBSD, since it does not rely on
chroot(2), and thus has somewhat different behavior.

May I suggest that you use the same interface, since that would be less
confusing for a user who happens to run both NetBSD and OpenBSD
installations ?