I had thought about this before for a setting specific thing too. I thought maybe as a house rule of using the Defense Rating of a Fighter/Striker/Seeker to simulate hits. I was modeling something that could take 3 hits so I thought if I gave it DEF 3 and marked off 1 for each point of DMG it took that should work. Tried it out  in a test game and it seemed to work ok, and as long as it was setting specific it didn't threaten to mess up the pointing of fighters in general. I actually think it "felt" better than giving a fighter flight a high DEF and seeing starship weapons hit it to no effect, which I just hate. I know, a high DEF could represent elusiveness or other survivability traits, but it just sucks to whack a fighter flight for several hits from your battlecruiser's big guns and see them all survive (ignoring high IMP and DMG to boot!) And yes, that's a true story 
Cheers,
Erik

Probably take everything up a level.

For example your ship might be 5 hull boxes, now make it 10

Have your Heavy torpedos as 1 hull ships using the ramming rules.

You can also have fighters as two hull ships.

This way you can even put defences on your torpedos.

My understanding was he was looking more for something that wouldn't be reduced by hits (until it reached its max and was destroyed), or have the turn ACC mods of the Plasma Torps either for that matter.
E

Two out of Four is not bad.

murtalianconfederacy does not say anything about damage requirement. Else I would say a single Seeker using your idea of DEF being used to take hits instead of defence.

The only other alternative I can see (at the moment) is to use a plasma torpedo weapon without the reduction or turn ACC mods and not allowing the weapon to be used as a direct fire (plasma bolt) weapon.

Paul