You could probably do that for now.
It'd amount to just a MonoBehaviour with a bool.
Then you set skeletonAnimation.skeleton.flipX
according to that bool on Start (not Awake).
We'll try to figure out where those fields fit in the official runtime and add it after 3.0. There's also the matter of skeleton color too.
Maybe we could add it on SkeletonRenderer itself, like how the new flip bools on Sprite Renderer do it.
It's an ambiguous API matter for the official runtime because it means the data is in two different places; One in the serialized MonoBehavior, and one in the Spine.Skeleton object.
Though we could also just call it initialFlipX and initialFlipY and call it a day.