Either you are vegan/vegetarian or you are an omnivore. I don't think it is any more unethical in raising geese for their liver than raising cattle or fowls or lambs for their fleshier parts. And in fact, there are less "cruel" ways to raise foie gras, more naturally, but it is going to drive the price up indefinitely that it is not economical to produce. The "dilemma" isn't unlike the one for free range cattle...
I've once tried some ethical foie gras (at my teachers house when she invited us over for a goodbye dinner) but I didn't like it, so you won't see me ever to buy it anyway.
I am sure there will be delicious but ethical foie gras out there, if not so already. Just that it wouldn't be affordable to most people since its "success rate" (in producing it) is SO low.