Netsuite PS actually did this for us during our implementation. It works ok, but you have to live with errors when saving the advanced pdf (it complains that it can't assign anything to the ?eval). Also the script we were using to generate the JSON was really slow. We ended up changing our upstream data extraction method instead for a cleaner advanced pdf Here's the "if chain" they used to get it to work:

<#if record.item?has_content>
  <#if record.customfield?has_content> 
    <#assign m = data?eval>
    <#list m["origItems"] as itemList>