I don't think this is quite what you were aiming for, but the first thing that comes to mind is something like:
Blood in the Water
When you fall into the Big Wet, roll +current-harm.
On a 10+, you like big hungry fish, right?
On a 7-9, pick one:
-something's coming soon, but it's isn't here yet
-you still have whatever you were carrying
-at least they're not big
On a miss, well aren't you a lucky dog.
I think you were hoping for something more like this:
Call it Motivation
The first time you Seize by Force after suffering harm, you may choose to roll +current-harm instead of rolling +hard. You may do this again after suffering further harm, but not until then.
Optional: Even if you don't have the advanced form of Seize by Force, you can still get the 12+ result when you roll +current-harm.
That doesn't solve the problem of having a potentially absurd bonus to the roll. But honestly, I think I'd like to see someone roll that +6 at least once, if only because it would mean that they were in deep shit and still sticking around to duke it out. Probably because they took the move in the first place :)
Of course, you might have had Blood Rage tickling at the back of your mind.
It says:
Whenever you take harm, roll +harm-taken. On a 10+, take +1 ongoing until you heal any harm. On a 7-9 take +1 forward. On a miss, take 1-harm or take -1 ongoing until you heal any harm, your choice
That one is on page 272 of the book, as a possible new character move, and was devised by Ben Wray.