Heh. Probably cost just or , as it's a pretty niche hoser. What does this do to Trap Runner / Curtain of Light, though??

It's not as niche as you think. Normally you have to Lightning Bolt opposing creatures prior to declaration of blockers; now you can wait to see the most effective way to use it.

I think it would hose your examples, but I'm not sure what order the effects are applied. Does a static ability reapply each time SBEs are checked?

If it works the way you want it to (and you can probably force it to) it's pretty useful - but certainly still niche. You have to be up against people who block-sac, or combo it with other removal. It does improve your removal though. Probably rare for its nichiness; probably still quite cheap. Certainly interesting.

Suggest you also have all unblocked creatures have flanking.

This seems functionally identical to "creatures you control have trample". A little worse, actually, seeing how as an ehchantment that did nothing but give your dudes trample would be borderline at best in limited and unplayable in constructed.

This is Ride Down in enchantment form, basically.