I got a better Idea Hobb, but it builds off of yours.
Instead of razing the fief to be useless, you can instead do it like this: Make it so that if you are within 1000m of a village(maybe a little more) you can "Raze the Country side". What this would do is Lock your character down for 5 hours while they perform the action. If you are attacked during this time window, then the devastation gets canceled and it turns into a field battle.
Now for the specifics, you should be able to raze 2 things, prosperity and population(growth?). If you decide to impact prosperity, any amount of troops would work, but for every 1 point of prosperity to be reduced, you need 2 soldier(this may need some tweaking and it might need to be 3 or higher if really needed). The time that this is lost should be between 4-7 days, though i think 5 days would be most suitable.
If you decide to kill of population you need at MINIMUM 1/3 of fief population. the amount of soldiers needed would be double that of the prosperity reduction. Does not affect army of fief.
Make this able to be done to your own fiefs as well, but at no penalty, Kinda like a scorched earth policy. If the enemy is going to take your lands, destroy your prosperity before they take it making it USELESS to them in the immediate term. Also make the prosperity reduction done to same faction fiefs last 7-8 days versus the 5 done to enemy fiefs.
Make the devastation cancel able at any point in the first 2.5 hours of doing it.
So for example:
I see a 300 Population village, 50 army and 250 S&D. I decide to Raze the country side with 100 soldiers. I choose to impact S&D. Provided I don't get attacked, the village will loose 50 S&D for 5 days meaning it would only have 200 S&D for the 5 day duration.
Using Same example above, I go for Population killing. With my 100 troops, and(in this example) 4 troops per 1 population death, I would kill 20 population. Of course the population WILL regrow and within 5 hours most likely be back to 300, but this way you can lower population before an assault to make it easier to take.
Final Example, I own a village and I realize the enemy is coming. My village has 200 prosperity and 300 pop. So with my 250 man army, I decide to raze the prosperity. Because it is MY fief the fief goes NEGATIVE prosperity(-50), thereby meaning S&D is LOST and the enemy knows that I or my faction will counter attack. If the war was over trade rights, I have effectively removed the enemies(and everyone else's) ability to trade there for 7 days.