Don't know, but I'm getting the impression that it's going to be a trim level. So F150 will have XL, XLT, Lariat, Platinum, Limited, Tremor, Raptor, Lightning. Although you could classify the BEV as an engine option...2.7L V6, 3.3L V6, 5.0 V8, 3.5L V6 Powerboost, Lightning....I don't think it will be.
For one thing, there are obviously rather significant differences between the Lightning powerplant and others, even from the hybrid. If it considered an engine option, I would think it would only show up on 1 or 2 trims at the most. But I still think it will be a new trim, probably more of a performance. If they want to market it as a work truck, or off road, then they could make a BEV option on the XL/Raptor/Tremor trim...or something....don't know.
I guess I'm assuming they are using the Lightning name because it's been used for performance before. But then again, the abused the Mustang name a little bit with the Mach-E.