Just put your data on some funny port, then? Or give it a long and hard
to guess name, that might actually have sufficient entropy to be any
use.

A less-than-16-bit "random" port is rather easy to guess.

And, if you really want to do port blocking, read the pf man page. It is
possible with a rule that adds IPs to tables. Perhaps after more than
one knock for "added security..."

In any case, I really don't see a need for OpenBSD to support these
kinds of silly things, the people who really want to do them can find
their own ways.