Ok, here's another version. In this one, you have three Scratches and
three Injuries. Each Injury you have gives you a -1 to die rolls for
relevant activities. If you would get Injured but you're already at
the limit of three, you're Incapacitated. At the end of a fight,
Scratches go away. Injuries stick around longer. 

Risk level goes like this:
(when talking about the 'first two' or 'first three' or whatever, 
 assume they go "scratch scratch scratch injury injury injury")

Terrible: if none of the scratch boxes are checked, check the first
Poor: if the first box isn't checked, check it; otherwise if the 
      first unchecked box is the third or less check it; otherwise nothing
Mediocre: if any of the first two boxes aren't checked, check them; otherwise
          if the first unchecked box is the fourth or less, check it;
          otherwise nothing
Fair: if any of the first three boxes aren't checked, check them; otherwise
      if the first unchecked box is the fifth or less, check it;
      otherwise nothing
Good: first four
Great: first five
Superb: first six
Superb+1: check all & incapacitate

Obvious point: if you get a good stab in early, then it's relatively
easy to scratch them to death later. Let's see how it goes with
Meg and Boffo:

Round 1: Meg Superb, Boffo Terrible. Meg's bonus goes to +4
Round 2: Meg Superb+2, Boffo Terrible. Meg cashes: Boffo gets two Injuries
Round 3: Meg Fair, Boffo Poor. Meg's bonus goes to +2
Round 4: Meg Superb+1, Boffo Mediocre. Meg's bonus goes to +3.
Round 5: Meg Superb+2, Boffo Terrible-4. Meg's cashes: Boffo gets a third Injury.
Round 6: Meg Superb, Boffo Poor. Meg's bonus goes to +3.
Round 7: Meg Superb+1, Boffo Poor. Meg cashes, Boffo is incapacitated.

Ok, that was reasonable, but the third injury made it take a while. On
the other hand, perhaps she should have raised the risk level. Also,
Meg got some lucky die rolls. Try again:

Round 1: Meg Fair, Boffo Fair
Round 2: Meg Good, Boffo Poor. Meg's bonus goes to +2.
Round 3: Meg Superb+2, Boffo Fair. Meg's bonus goes to +3.
Round 4: (Meg raises the risk level by 2) Meg Great, Boffo Mediocre.
         Meg cashes: Boffo gets three Injuries.
Round 5: Meg Fair, Boffo Terrible-2. Meg's bonus goes to +3.
Round 6: Meg Superb+3, Boffo Terrible-3. Meg cashes, Boffo is incapacitated.

That's a little better.

Meg vs the long-suffering Claude:

Round 1: Meg Good, Claude Good.
Round 2: Meg Great, Claude Great.
Round 3: Meg Good, Claude Great. Claude's bonus goes to +1.
Round 4: Meg Good, Claude Superb+2. Claude's bonus goes to +3.
Round 5: Meg Fair, Claude Superb. Claude's bonus stays at +3.
Round 6: (Claude raises the risk level by 1) Meg Superb+1, Claude Good. 
         (I dunno, I just roll 'em) Meg's bonus goes to +2.
Round 7: Meg Superb+1, Claude Superb. (note that there's less
         incentive to cash here, unless you think he is likely to come
         back; if you cash he'll be at -1, whereas if you raise your
         bonus to +3 and cash he'll be at -2 when you cash)
Round 8: Meg Superb+3, Claude Superb. Meg's bonus stays at +2.
Round 9: (Meg, getting bored, raises the risk level by 1) 
         Meg Great, Claude Good. Meg cashes: Claude gets two Injuries.
Round 10: Meg Great, Claude Great.
Round 11: Meg Good, Claude Fair. Meg's bonus is +1.
Round 12: Meg Superb, Claude Good. Meg's bonus is +2.
Round 13: Meg Superb, Claude Mediocre. Meg cashes: Claude gets a third injury. 
Round 14: Meg Poor (!), Claude Fair. Claude's bonus is +2.
Round 15: Meg Good, Claude Poor. Meg's bonus is +2.
Round 16: Meg Good, Claude Terrible-2. Meg cashes: Claude is incapacitated.

One more theory: only five boxes, three scratches and two
injuries. having three scratches is worth -1. having an injury is
worth -1 per. since we're down to five, you only need a superb risk to
incapacitate.

Round 1: Meg Great, Claude Poor. Meg's bonus is +3.
Round 2: Meg Superb+1, Claude Superb. Meg's bonus stays at +3.
Round 3: Meg Superb+4, Claude Good. Meg's bonus goes to +4.
Round 4: Meg Superb+2, Claude Superb. Meg cashes: Claude gets all his
         boxes checked.
Round 5: Meg Superb+2, Claude Poor. Meg's bonus goes to +4.
Round 6: Meg Superb+3, Claude Terrible-1, Meg cashes: Claude is out.

Another:

Round 1: Meg Superb, Claude Great. Meg's bonus is +1.
Round 2: Meg Fair, Claude Good. Claude's bonus is +1.
Round 3: Meg Good, Claude Superb. Claude's bonus is +2.
Round 4: Meg Fair, Claude Superb+2. Claude's bonus is +3.
Round 5: Meg Great, Claude Superb+2. Claude cashes: Meg gets one Injury.
Round 6: Meg Mediocre, Claude Superb. Claude's bonus is +2.
Round 7: Meg Poor, Claude Great. Claude's bonus is +3.
Round 8: Meg Great, Claude Superb+1. Claude cashes: Meg gets another Injury.
Round 9: Meg Poor, Claude Fair. Claude's bonus is +2.
Round 10: Meg Fair, Claude Great. Claude's bonus stays at +2.
Round 11: Meg Terrible-2, Claude Superb+1. Claude's bonus goes to +4.
Round 12: Meg Good, Claude Superb+4. Claude cashes: Meg is out.

That took a ways longer. Perhaps if Claude were smarter he would have raised 
in round 8 or 9.

And drag out Boffo for one last fight to cheer Meg up:

Round 1: Meg Superb, Boffo Good. Meg's bonus is +2.
Round 2: Meg Superb+2, Boffo Mediocre. Meg's bonus is +4.
Round 3: Meg Superb+4, Boffo Poor. Meg cashes: Boffo is Injured twice.
Round 4: Meg Good, Boffo Terrible-1. Meg's bonus is +3.
Round 5: Meg Superb+1, Boffo Terrible-3. Meg cashes: Boffo is out.