I really doubt you can add enough 'ballast' to make an appreciable difference in how an optimist would behave without adding enough weight to seriously compromise the boat, esp in the event of an accidental gibe/wipeout.
Also if flipped now these weighted boards could slide towards the hull making righting from the water more difficult.
I think both Paul of NWOnt and ftldiver are talking about the same thing... turning the opti into a trimaran.
My understanding was that Optis are already one of the most inherently stable training dinghies.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)