If I am right and the problem is caused by a UNICA seatpost that is too short, then no amount of reinforcing the rear seatlug (short of welding on 1" of rebar, perhaps) will solve this problem, as long as the seatpost is up higher than it deserves to be.
If the problem was caused by some sort of weakening or trauma to the seat lug, then a washer or some other filler or reinforcement of the seat lug would be an appropriate solution.
- Don Gillies
San Diego, CA, USA