umm I don't know if there is a slang for it, since not many people have that in Japan compare to other western countries. So if I say stretched earlobes in Japanese it would be 拡張ピアス(かくちょう ピアス/kakuchou piasu). 拡張(かくちょう /kakuchou=enlarged)ピアス(piasu= pierce). Or ホールピアス(hooru piasu) ホール(hooru＝hole)ピアス(piasu= pierce).

I've seen the accessories for sale with: ストレッチピアス or just simply ストレッチング As said by Rieko-Japanese, 拡張 is also used. I'm not sure of a slang term though.