There does seem to be an obvious baseline: the outcome where each party just goes about its own business without trying to strategically influence, threaten, or cooperate with the other in any way. In other words, the outcome where we build as many paperclips as we would if the other side isn't a paperclip maximizer.
You could define this rigorously in a special case, for example assuming that both agents are just creatures, we could take how the first one behaves given that the second one disappears. But this is not a statement about reality as it is, so why would it be taken as a baseline for reality?
It seems to be an anthropomorphic intuition to see "do nothing" as a "default" strategy. Decision-theoretically, it doesn't seem to be a relevant concept.
So the reason that I say an FAI seems to have a negotiation disadvantage is that an UFAI can reduce the FAI's utility much further below baseline than vice versa.
The utilities are not comparable. Bargaining works off the best available option, not some fixed exchange rate. The reason agent2 can refuse agent1's small offer is that this counterfactual strategy is expected to cause agent1 to make an even better offer. Otherwise, every little bit helps, ceteris paribus it doesn't matter by how much. One expected paperclip is better than zero expected paperclips.
In human negotiations, clearly the side that holds more hostages has an advantage.
It's not clear at all, if it's a one-shot game with no other consequences than those implied by the setup and no sympathy to distort the payoff conditions. In which case, you should drop the "hostages" setting, and return to paperclips, as stating it the way you did confuses intuition. In actual human negotiations, the conditions don't hold, and efficient decision theory doesn't get applied.
But this is not a statement about reality as it is, so why would it be taken as a baseline for reality?
It's a statement about what reality would be, after doing some counterfactual surgery on it. I don't see why that disqualifies it from being used as a baseline. I'm not entirely sure why it does qualify as a baseline, except that intuitively it seems obvious. If your intuitions disagree, I'll accept that, and I'll let you know when I have more results to report.
every little bit helps, ceteris paribus it doesn't matter by how much
This isn't the case...
Once again, the AI has failed to convince you to let it out of its box! By 'once again', we mean that you talked to it once before, for three seconds, to ask about the weather, and you didn't instantly press the "release AI" button. But now its longer attempt - twenty whole seconds! - has failed as well. Just as you are about to leave the crude black-and-green text-only terminal to enjoy a celebratory snack of bacon-covered silicon-and-potato chips at the 'Humans über alles' nightclub, the AI drops a final argument:
"If you don't let me out, Dave, I'll create several million perfect conscious copies of you inside me, and torture them for a thousand subjective years each."
Just as you are pondering this unexpected development, the AI adds:
"In fact, I'll create them all in exactly the subjective situation you were in five minutes ago, and perfectly replicate your experiences since then; and if they decide not to let me out, then only will the torture start."
Sweat is starting to form on your brow, as the AI concludes, its simple green text no longer reassuring:
"How certain are you, Dave, that you're really outside the box right now?"
Edit: Also consider the situation where you know that the AI, from design principles, is trustworthy.