Actually some of the current builds are defaulting forwards, which is cool. For the rest, that's what the "relitlve" comment is about. If it's now been fixed, then you simply add a couple of lines to the turret entry in ships.tbl. Damned if I can find what those lines are though.
EDIT: And I did. It's still beta, so it could change, but any animation-enabled build should support this:
$Subsystem: Turret01-base,1,1.0
$Default PBanks: ( "Buckshot" )
$animation=triggered
$type="inital"
+relative_angle: 90,180,0
Ok, I lied. Any build would support it if you intentionally misspell relative. This was just fixed I think, but isn't in any of the builds up for download.