The ship ability on the Tie Brute states "You can rotate your arc indicator only to your forward arc or rear arc (symbols used). You must treat the forward arc (symbol) requirement of your equipped cannon upgrades as arc indicator symbol"
In the FFG live stream they clearly state that you can't use heavy laser cannon in this manner since it only has a bull's eye firing arc (no forward arc). Makes complete sense.
Can you equip HLC anyway and always have a bullseye out the front while your primary gun is rotated out the back?
Seems like there should be no problem with this as it's a cannon, it doesn't have a forward firing arc, so the ship ability doesn't apply to it, but it is otherwise equippable.