Tony Taylor wrote that on a butted frame would not an extra long seat post be of little help, as suggested by Don Gillies for added support.
Not all cases, but almost all cases, the seat tube is single butted, only at the BB shell should be the butting, from time to time this tube gets inverted.
On the Cinelli Pista in question, the typical Cinelli has an internal reinforcement of about 85 to100 mm in length, give or take, one reason Cinelli's typically use 26.2 and not 27.2 seat posts. As a result, this area has already been well heated, maybe why the crack started other than a gorilla mechanic.
The "silver braze on a washer" or equal reinforcement to arrest the current crack with a drilled hole end and opening up of the crack area down to that drilled hole is probably a sound solution. I have done similar repairs, never to see the frame again.
The Don Gillies solution is not applicable to Cinelli's way of doing things in this era.
On this Cinelli Pista in question, I would check the depth of the internal reinforcement, just to know what is all at play.
John Jorgensen
Torrance CA USA