May one break a minyan to go eat Shalosh Seudos before it gets too late?

It is likely as many shuls and people do, that he ate a Kiddush during the day, so in a sense he ate Bedieved or after the fact already Shalosh Seudos. Otherwise, he may just grab in shul a large enough piece of Mezonos and promptly comply without leaving the shul without a minyan.