Ditto Tael. Feudal relationships, at least in theory (a theory which people in the Dark Ages took very seriously, as seriously as we take all our theories about democracy and the free market and socialism and whatnot), involved rights in both directions, though always asymmetrical. The serf owes the lord labor, the lord owes the serf protection. If the bandits are raiding the peasantry, while the lord sits in his manor hall feasting and making merry, the most doughty of the peasants can stride into the firelit hall and Claim His Right: "great father, how much longer can you ignore our suffering? are you our protector to whom we look, or are you not?"
One thing that's way better about the move than Go Aggro is that the +1 means the follow-up is flexible. It's perfectly clear that the serf's next move is not Go Into Battle. But if the lord, his face turns purple, his fist clenching around the gravy-soaked drumstick, stands up from the table and shouts "fool of a villein! how dare you question me! Hargis, Vinter -- seize this worthless cur and toss him in irons!", the serf has +1 forward. His next move might be Draw Someone Out, asking the lord's player, "how can I get you to commit your troops to defending the peasantry?" -- or, perhaps Draw Someone Out on the lord's brother-in-law who is silently observing and is known to covet the lord's land, locking eyes with him as if to say "how can I get you to invade? you see that the people are with you!" Or, his next action might be to take stock, take his bearings, pray, or hold steady.
That said, the lack of any Manipulate type move does seem to be somewhat glaring here. What if the peasant's next action is to incite a rebellion? It seems very natural that your having Claimed Your Right vis-a-vis the lord should give a +1 to a roll to do this, but to what roll? As written, you can spend it to get information which might help, but that's about it. Actually going among the people and saying "we've had enough of this, it's time to get our pitchforks and storm the manor" (something that happened quite a bit in the middle ages, at least) doesn't seem to have an obvious mechanical hook. Hold Steady doesn't seem to fit the way that even Act under Fire would -- as the OP noted, it's less of a catchall.
There's nothing wrong with it just being in the fiction (or left to playbook moves, for an appropriate class), it's just sort of surprising.
"Okay, I gather my friends at my barn, and tell them that it's time to rebel against the vile baron!"
"Well, that doesn't trigger a move." Everyone looks at the MC, who makes an MC move.
Alternatively, I guess gathering the friends at the barn could be ALSO Claiming Your Right. "Too long have we suffered at his hands -- I call upon you, friends, brothers, by our rights as ancient dwellers in this land before the usurpers came..." So then the whole community of peasants is "your counterpart" and on a 10+ they can choose to call you a fool and you have +1 to ask the MC "how could I get them to rebel..."