I have seen that some sing Shoshanas Yaakov on Pesach to commemorate the death of Haman. Can someone please direct me to an authoritative source for this custom?
Taame Haminhagim 579 says Sh'ne Luchos Hab'ris [circa 1600] indicates one should memorialize Ester's meal at the meal of the second day of Pesach, as that was the day Haman was hanged. He doesn't say how to memorialize it, but I guess Shoshanas Yaakov is one way to do it.
Update: Thanks to Aba, who found the Sh'lah (in the hagaha on the linked-to page). Note that it doesn't use "oughta" language like the Taame Haminhagim: it says, rather, that some who are careful do so. And it adds (as the Taame Haminhagim doesn't) that the purpose of this practice is to mention the praises of God [which makes the singing of Shoshanas Yaakov seem to me to be a good implementation of this practice].