I can't see any production information about this, but I think there is strong circumstantial evidence that if Leonard has a voice actor, that voice actor may be Majel Barrett.
Leonard the iguana actually has two lines. "Jerks" as you say, at 27:20, and one earlier line "Just being thorough" at 16:20.
The voice is clearly manipulated in some way, and if you speed the track up by 25-30% it sounds much more natural, although delivered in a flat affect. (it does not appear to be the original delivery of the actor being parroted (Robert Picardo or Marina Sirtis), which was my first assumption.)
SAG rules prohibit production staff from acting as performers except for Writers, Directors and Producers in certain situations. (eg Section 67 of the 2014 SAG-AFTRA master agreement for TV productions. Obviously that agreement was not in place in 2000 when the episode was made, but similar rules would have been in place) There's no exemption for minor parts or because it was manipulated.
Based on these two facts, I think there is no reason to believe that the producers would have had anyone other than the performers that we know worked on the episode record these lines.
(To my ear, the sped up voice also sounds like Barrett, but without knowing how much it was slowed down by, and given how short the lines are, it's hard to credit that. The lines are also too short to definitively rule out Picardo or Sirtis, either. While they don't seem to quite match up with their audio in the episode, the sound editors would have had access to multiple different takes to start from.)