He would have to attach it "below" the merchant's tx, but yes, you're right, it's a possible attack vector. Anyhow, the referencing algorithm is not yet finished, so we are discussing it with CfB right now.
Possible? I don't see how it's possible to draw a picture to have longest-path-as-the-score rule to be broken by an adversary.
Anyhow, let's finish our private discussion in slack and only then make the results public.