I am about halfway thru Onion Movie and have noticed a lot of actors who were on Seinfeld... "Mr. Morgan," the security guard from the finale, "Duncan," "Mr. Lipman" among others...
shareThere's no connection because Seinfeld is actually funny.
shareThere were, by my count, eight Seinfeld actors.
In addition to the 3 ones you mentioned: Mr. Lippman (Richard Fancy), the prison guard from the finale (Scott Klace) and Mr. Morgan (Tom Wright), there was:
*Don McManus (Jerry's old classmate whom he defeated in The Race)
*Ken Takemoto (Offended Japanese executive who had his handshake declined by Mr. Lippman)
*Barbara Pilavin (German tourist duped into thinking George was being mugged by Kramer)
*Gedde Watanabe (One of the Japanese tourists whom Kramer befriended)
*Daniel Dae Kim (Medical student)