The War Caster feat (PHB, p. 170) allows a single-target spell ("must target only that creature") to be cast instead of an opportunity attack.
The Green-Flame Blade cantrip (SCAG, p. 142) appears to have a single target, and then some damage "leaps" onto a creature 5 feet away. The other creature is never referred to as the target, or even "a target".
Can the Green-Flame Blade cantrip be cast as a reaction using the War Caster feat?
I'm pretty sure this is solid from the RAW perspective. Is this legit from a RAI perspective?