Can a married couple claim 50 embryos as tax deductions, for example?
Under current law, probably not. But ideally? If they're paying bills to keep those embryos intact, then yes, proportionate to said expense (non-refundable).
Can you use the HOV lane if you're pregnant?
Ideally the rules should be applied the same as when a woman is driving with one baby in her car. Provided, of course, that she can give some valid proof of pregnancy.
If someone who has a child successfully through this method then leaves 40 embryos behind, are they legally responsible for paying for the cryogenic storage of same?
Ideally they ought to be, yes. If they didn't want to assume this risk, they should've went with adoption instead.
If they refuse to pay for this, are they now criminally liable as negligent parents for lack of care?
If their refusal leads to avoidable embryo death then yes, ideally they should be charged the same as any parent whose baby died from neglect.
Do the embryos become wards of the state, supported by tax dollars, if abandoned by parents, or if the parents die, knowing they will be stored indefinitely?
Ideally yes, they would be, with the state paying women to become surrogates. And ideally this expense would lead the state to ban IVF or only permit advanced techniques which don't create excess embryos.
What's the current thinking?
My thoughts are above. If you're trying to pull off a "gotcha" that the laws of the land haven't caught up to the practical implications of embryos being legal persons, then it's only a matter of time, assuming that good faith pro-life legislators eventually triumph. But for the time being, the tax status of embryos is pretty low on the list of issues we care to focus our attention and efforts on.