Here's the answer! I know it's a little unorthodox to answer the trivia myself, but I'm going to be away for a few days without access to the Web (yes, you can get away, really away, far, far away ... until the black flies drive you back to civilization).

Anyway, the poet is Catullus, the reference to Mamurra as Caesar's mentula diffututa ("drooping penis") can be found in Poem 29. Mamurra was an engineer with Caesar's legions in Gaul. He was an equestrian from Formiae and for whatever reason Catullus disliked both Mamurra and Caesar enough to suggest that they were lovers.

Sorry I had to do that. Didn't know I was going canoing. See you in a few days.