It was cirtainly done before Tarrega's time .Regondi (1822-1872) ,in his Reverie (Nocturn) Op 19 .When he composed the piece I can't say.Neither
can I say for cirtain wether he was the first to introduce the effect after the CG guitar was revived by Sor and Giulianni and the other Romantic Composers.
It would be interesting though ,to discover who was the first to introduce the effect after the revival of the CG guitar ,as we know it today.I doupt wether it was used on the Lute or Baroque Guitar.For those who havent heared the above mentioned piece here is Iona Grandrabur doing it justice.The tremolo section comes after the Larghetto Section so you are going to have to be a little patient.
http://www.youtube.com/watch?v=-7iFUgc2bhc