I think that the first part of the quoted sentence ("Southey liked a
roasted rat for supper") is a witticism based on an old saying
"rhyme (Irish) rats to death" (see Brewers' Dictionary of
Phrase and Fable, 1894, with examples from Ben Jonson, Sidney and
Shakespeare), that is to write monotonous, endless verses--exactly what
Southey did in his notoriously long epic poems. The implication is
transparent: Southey rhymed so many rats to death because he liked them
for supper. Kinbote wants to say that T.S.Eliot's poems are as boring
and
monotonous as those of Southey, once a very influential and highly
respected poet-laureate.
Alexander Dolinin