Good points, all!
Here's a revision -- does this reflect how you'd like to see them repped?
TRANSFORMATION / BOARDER (x1.33): At the beginning of the fighter phase, this flight may declare that it is transforming/boarding.
During the following round, each ship which successfully hits and penetrates the enemy vessel do not roll on the damage table as normal. Instead, the fighter is translated into a single Marine squad on the enemy ship. This squad has boarded the enemy ship and may fight in a boarding action THAT ROUND and all following rounds.
Fighters in the flight which fail to damage remain in play as normal, and may attempt to transform/board again next turn.
At the beginning of any subsequent fighter phase after boarding, a marine squad may declare it is disengaging/rembarking. Each surviving marine squad becomes one fighter in a flight, which may act in that fighter phase.
The end -- I hope this covers it. Oh, and in VBAM, this has fun potential, especially if you're using the 'detailed ground combat' rules.
-Adso
<sound of cackling as he ponders Heavy Bomber Transformable boarding mechs...>