From the latest FAQ, as well as the 'large' ship rules, we see:
Some ships (such as the Slave I ) have an AuxiliAry firing Arc identified by dotted lines printed on its ship token. These ships also have a unique primary weapon icon printed on their Ship cards. When attacking with its primary weapon, a ship with this icon may attack an enemy ship that is at Range 1–3 and inside its standard or auxiliary firing arc.
(Emphasis mine)
Yet, looking at the Scum cards...

...the Firesprays lack that icon.
I know this was brought up a while back, when the pre-production art was being circulated, but...I just checked my retail box parts, and sure enough, the Scum Firesprays lack that icon.
Sooo...I guess someone being a real 'rules lawyer' could insist that the Scum Firesprays cannot fire in reverse at the moment. The rules do say flat-out that the dotted fire arc AND ICON is required to have that ability, and the Scum Firesprays don't have that icon. This has the potential for serious YUCK at high-level play...