On the Item Demand Plan screen, there is a sublist that is only accessible on the page. I can load the record and see the lines (Demand), but I only see the default listing, which is filtered to this year. Is there any way to modify the list for a different interval? (Through SuiteScript, server side, I mean. I see that the “year” or date field filters are editable in the UI/currentRecord module)

you can try sharing your code, but the general advice is use dynamic mode and use matrix fields

WILD! See this: https://netsuite.custhelp.com/app/answers/detail/a_id/35055/kw/item%20demand%20sublist