How does this work if at all?

Although the GR75 has no fire dice if you use Opening Salvo do you get the 2 dice as a one-off shot?

You gather your dice together and hey presto you have none so you cancel the attack before you add the opening salvo dice.

To expand a little (before Dras does)...

From page 2 of the RRG, the attack sequence is:

1. Declare Target

2. Roll Attack Dice

3. Resolve Attack Effects (which includes adding the dice from Opening Salvo)

However, before you reach step 3, there's the last bullet point of step 2:

2 . Roll Attack Dice: Gather attack dice to form the attack
pool and roll those dice. Gather only the dice that are
appropriate for the range of the attack as indicated by
the icons on the range ruler.
◊◊ If the defender is a ship, gather the attack dice
indicated in the attacking hull zone’s or squadron’s
battery armament.
◊◊ If the defender is a squadron, gather the attack dice
indicated in the attacker’s anti-squadron armament.
◊◊ If the attacker cannot gather any dice appropriate for
the range of the attack, the attack is canceled.

So you never get the opportunity to modify the dice in the attack pool (which happens in step 3), since the attack is cancelled during step 2.

Thanks, thought that was the case but always best to check with the oracle................

Note that I assume you mean the medium transports, as opening salvo could be a special kind of terrifying with a bunch of combat refits.