upon the creation of a sales order, I'd like to set the inventory details on the line item - I believe the inventory details is a subrecord. could we do something like this?

I think you can only set the inventory details when you fulfill the order and there is a sample snippet on the help guide to set line level inventory detail.

What kind of script? If its client side, that is not possible as far as I know. You can set the Inventory Details in the SO, its just not required like in Item Fulfillments or Invoices.