[Suicide Junkie]

Or a toxin injector missile, perhaps.

But I wouldn't worry about adding one point of boarding defense to a few components.
The attackers can always just include one more boarding party.

BTW, you will probably want to add the defense ability to boarding parties as well. They currently will provide defense just like the crew quarters without being hit by Boarding-defense-only weapons.
You can comment on the "Home turf advantage" of the boarding parties to explain it if you want.