So, this ended up working. 

I'd taken parts of stuff from:
https://github.com/huskyproject/hpt/blob/master/misc/vtrack/vtrack.pm

Mostly the "do some aliases", the sub me, and that's it so far.

Then in my own filter.pl, I require my crafted functions.pl in the BEGIN {...}
section, in my filter sub, I do my usual condition and just post it back
through like this:

if (!defined $toarea && $toaddr eq "1:135/300" &&
$toname eq "Eric Renfro") {
  post('NETMAIL', $sfrom, $sto, $from, '1:135/371', $subj, $date,
($attr|TRS|K_S)&~(LOC|ATT|FRQ|URQ|KFS|TFS|HLD|CRA|DIR|IMM|ARQ|RRQ), $text, 3);
}

It retains both the original that was sent, and re-posts the new one as a copy
and gets packed and delivered to me on my BBS node as I'd hoped, with the same
from and to, just different toaddr.

)))[Psi-Jack -//- Decker]
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