If the desire for the claim power level changes is to introduce a period of vulnerability to promote siege, have you considered a world even similar to the meteor?
A dark moon, or conjunction of sorts, that strains the sanctity of all fortifications? An event notification would appear upon log in with a warning that it will happen in the coming days.
If the desire is also to give the defender an advantage by selecting a time window that is favorable to them, you may also consider a mechanic (perhaps another use for Numen points) in which claim owners/lawspeakers can pray to set a specific time that the event will affect them, within the window.
I think it would be interesting if attackers had to balance the prospects of their raid against their own vulnerability (assuming the event would be a 24 hour window, or maybe a few days long, with the individually set vulnerability windows in the 2-8 hour range). Large factions having to deal with the potential threat from other large factions may also draw their focus away from the casual hermit.