Is there any kind of penalty enforced by either Y'ffre or the Bosmeri authorities?

Eaten.

For enjoying some fruit? Damn, glad I'm not a Bosmer

Well, I doubt anything metaphysical happens... I imagine they're spurned, hated, and kicked from society.

Assuming, of course, this is a plant from Valenwood. Foreign stuff is okay, though I imagine some orthodox people dont eat/use/harm/hinder any plant-life from anywhere.

I'm half joking. I'm not sure its ever really stated, but it wouldn't surprise me if thats the punishment. Otherwise I'd go with the statement above.