How would you treat a cancer of undetermined primary site presenting as peritoneal carcinomatosis, with pathology suggesting intestinal or pancreatobiliary origin?  

Assuming the rest of the work-up for unknown primary/GI primary is unremarkable, I would probably treat with FOLFOX. That would cover all the basis as well as possible. Depending on whether there is any organ involvement, I would also consider surgical evaluation for HIPEC (but only in the healthy, ...

I assume that the patient has had an EGD and colonoscopy that are negative. If not there is value to determining wher ethe primary is both to determine chemotherapy choices and to anticipate potential local issues related to the primary site. If that has been done and is negative then, as @Yous...