You can get some special threaded brackets made for the mounting bolts on
seat pedestals. Same idea as toggle nuts, that you can stick thru the
holes
and hold in place w/ a plastic link until the bolt is screwed in.
Otherwise,
flip the boat over and inject some thickened epoxy into the holes and
gravity will keep it in place.

I don't suppose you can reference that in an on-line catalog, can you...

It would be easy enough to put the boat up on sawhorses to fill the holes;
that is a good idea.