Does X work to modify Snap Shot? Does Y work to modify against OL when the ship is locked?
It occurred to me when chatting with friends that there seems to be a conceptual way to think about these things that makes it pretty simple. It comes down to this:
Pretend that A -- the ship with X or Y -- is played by a separate human being from B -- the ship getting modified by X or Y. Same team, but separate people.
Then ask yourself, "Does B have to have A's permission to modify this roll?" If the answer is "yes," then A is the source of the modification. If no, then B is the source of the modification.
So take Howlrunner. Howlrunner is played by A, Mauler Mithel is played by B. (Same team, different people.) Does A have to do anything to give B (Mauler) access to Howlrunner's re-roll? No. Howlrunner simply has an "aura," and if Mauler is inside that aura, Mauler can benefit. Therefore B (Mauler) is the source of the modification. Therefore it can't be used against, e.g., OL, if OL has Mauler locked.
Now take M9-G8. Poe with M9-G8 is played by A. Tycho with Snap Shot is played by B. (Same team, different people.) In this case, does A (M9-G8's owner) have to do anything to give B (Tycho) the benefit of the modification? Yes, A must select the die and tell B to roll it. Therefore, A is the source of the modification, and since Snap Shot only forbid Tycho from modifying dice, it works for Snap Shot.
I haven't exhaustively checked this conceptual framework against every possible card, but I haven't found a counter-example yet. It basically comes down to an active directive to modify versus an "aura" that allows modification.