Okay, just one more question (that may be a lie), "You cannot modify your attack dice and cannot attack again this phase." Does the you refer to you as the player, or you as the ship (considering it then refers to them as 'your' attack dice)? If it's you as the ship, then it would make sense that things like Jonus and Palp could modify the dice, as those aren't effects on that ship. However if it does mean you the player, then the opponent could theoretically modify the dice through Sensor Jammer/Elusiveness/pilot abilities/etc. It would seem quite lackluster if you can't use any effects to modify but the opponent can.
Assuming that they are on another ship, Palp or M9-G8 would be able to modify a Snap Shot attack. Jonus would not as the ship benefitting from his ability counts as modifying its own dice.
Snap Shot
After an enemy ship executes a maneuver, you may perform this attack against that ship. Attack: Attack 1 ship. You cannot modify your attack dice and cannot attack again this phase.
M9-G8
When a ship you have locked is attacking, you may choose 1 attack die. The attacker must reroll that die.
You can acquire target locks on other friendly ships.
The attacker would be the ship re-rolling/modifying the die and therefor could not trigger M9-G8 when using Snap Shot. The ship equipped with M9-G8 chooses which die to modify but the ship targeted by M9-G8's ability is the one doing the modification.
Interestingly, if you used M9G8 to PENALISE an opponent's snap shot, it woudl work, per the rulings around OL and R7 Astromech.
But I'd agree that it wouldn't work to buff a friendly Snap Shot. The only thing that can do that by dice modification is Palp.
M9-G8 on a different than the ship with Snap Shot should definitely allow modifying Snap Shot attacks. The Omega Leader FAQ regarding R7 should apply here. That ruling has the ship with the upgrade as the source of the modification not the ship that is actually rerolling the dice.