I just purchased my first Catamaran, it's a venture 15 with bananna hulls. It's in fairly good condition for the year (1974) It looks like the hulls have been patched a couple times. Anyway... on to your problem... I am not an expert by any stretch of the imagination, however... If it were me this is the way I would repair
1- Drill out the hole so it's big enough to fish out the nut that dropped into the inside of the hull. (use a teliscoping magnet thingy to retrieve it... or one of those mini claw thingies.) This is the olny way I can think of to get the nut out short of cutting a hole in the hull, which I personally wouldn't do.
2- You need to purchase a stainless steel bolt that's long enough to reach through your trampoline mount and hull.
3- tie some fishing line
to the threaded end of the bolt and lower it into the hull keeping the line
from dropping into the hull as well. Then thread a piece of aluminum (or stainless steel if you prefer) bar stock that's been drilled to fit the bolt onto the line
and feed it into the hull. Once the bar stock has dropped into the hull, lift the line
and manipulate it till the bar stock dropps onto the bolt sliding down to the bottom of the bolt forming a "T". Lift the bolt back through the hole until the bar stock stops it. (your bar stock needs to be long enough to form a strong stop.) At this point if your bar stock has a good seal with the interior of the hull you can simply drizel some fiberglass resin into the hole and alow it to harden around the bolt. (you'll need to make sure your bolt is straight in the hole, if it's crooked you won't be able to mount your tramp to it.
Option B cut a hole in the deck and install an "inspection port" (a round threaded piece of hardware that should be available at a sail shop.)
Option C call McGregor and see if they have a solution for you.