Hello! We have a customer who requires (I won’t go into why here) that we invoice in different currencies per orderline. As far as I understand, that is not possible within Netsuite’s standard limit. But has anyone accomplished this with a customization or a workaround?

Not possible for a number of reasons but you may be able to group invoices in different currencies, can't remember if you can do that. It is not a problem to print some conversion based on logic but the transaction itself cannot have different currencies

If it is just for display to the customer you could edit the PDF and do some custom field setup on the lines of the transaction to reference a foreign currency. Seems clunky and in reality your transaction would only be in a single currency