Having had a good look at the small crack behind the seat lug on Nathan's Cinelli Pista, and viewing it from a frame-builder's point of view, I would subscribe in part to the solution that Keveni has put forward....something such as silver-solder needs putting in that crack.
I am assuming that Kevin's idea of soldering on a washer, implies that the washer would be soldered on to the tube below and probably touching the bottom edge of the lug and that the slot would then be lengthened into the washer. This would work very well, the only disadvantage being that the existing hole would bulge out slightly beyond the straight edge of the slot.
I think that I would take the washer solution a little further. In the first place I would open up the crack by filing it into a "VEE" section. This would have the advantage of producing new clean metal and also gaving a greater surface area for the silver solder to adhere to. That joint would be done with a slightly higher melting point silver solder.
I would then measure carefully, with a pair of dividers ,from the centre of the existing hole to the bottom edge of the lug..perhaps 6 to 8mm. thereby obtaining the radius of that circle. I would then find a new washer whose diameter was equal to twice that radius and which also had a hole through its centre the same diameter as the existing hole on the lug.
I would remove by filing away the upper edges of the washer until the remaining metal fitted snuggly and unobtrusively over the bottom of the lug, with both holes sharing the same centre. This larger washer would be bent to match the curve of the lug and silver-soldered into place using a solder with a slightly lower melting point..ie carry out a two-stage silver-soldering process. This would result in the seat lug itself being both soldered up and reinforced.
If carried out with a modicum of skill, the repair could be made to look like an authentic original improvement of a standard lug.
However, if you dont want to go to this amount of care and skill, you could always take the frame to your local auto body shop and have the welder there, lay down a 6mm length of TIG. bead to seal up the crack ...then file it smooth.
Norris Lockley
Settle UK