question about sales orders / data model: is there an (out of the box) way to manage/represent related line items on an SO? for example, if there is a warranty that can be associated with multiple inventory items, what method/field/whatever (in terms of the suitescript api) am i looking for to add a line item that specifies "warranty purchased for this specific line item"?

For this you may want to consider a specific handler on new object where you can then implement various strategies. Not out of the box, but eloquent.