What's the easiest way to get the lineuniquekey of a specific item from a purchase order to set it on the line of an inbound shipment?

What kind of context are we talking about, i.e. what script type, entry point, and event type?

Probably a search, depending on the answer to ^

I need to do this purely in a suitelet

your options are to do a search, or load the record and get the lineuniquekey from the sublist

You can also use a query on the transactionline table