why might the reference to philomel be significant?

The reference directly relates to the novel's theme of lust;

Perhaps the most famous episode in "The Waste Land" involves a female typist’s liaison with a “carbuncular” man. Eliot depicts the scene as something akin to a rape. This chance sexual encounter carries with it mythological baggage – the violated Philomela, the blind Tiresias who lived for a time as a woman. Sexuality runs through "The Waste Land," taking center stage as a cause of calamity in “The Fire Sermon.” Nonetheless, Eliot defends “a moment’s surrender” as a part of existence in “What the Thunder Said.” Lust may be a sin, and sex may be too easy and too rampant in Eliot’s London, but action is still preferable to inaction. What is needed is sex that produces life, that rejuvenates, that restores – sex, in other words, that is not “sterile.”