Can I use opportunist to roll an additional attack die with a secondary weapon, like with an HLC?

Yep! Same for Jan Ors. So long as Opportunist or Ors (Pilot) would be still be valid options anyway, then yes they can bolster a Heavy Cannon.

One of the local guys I play with had an amazingly frustrating list with Ibtisam + HLC + Opportunist, alongside Jan Ors and VI Wes Janson. That 6-dice gun hit like a brick shed.

Opportunist specifies an attack, no other qualifiers. So yes, it works regardless of the weapon you use.

That 6-dice gun hit like a brick shed.

Shouldn't it be 5-dice? HLC is 4 + 1 (opportunist) = 5...you don't get the range bonus with secondary weapons.

That 6-dice gun hit like a brick shed.

Shouldn't it be 5-dice? HLC is 4 + 1 (opportunist) = 5...you don't get the range bonus with secondary weapons.

4 from HLC

+1 from Opportunist

+1 from Jan

So, we could have Nera, using an APT (5), plus one for Jan (6), plus Opportunist (7) in a 360 arc...Love it

Yep