@Ant3000 Just had a quick call with the CTO at SparkLayer. It sounds like at the moment, it’s not possible at all to add line item level custom fields or properties, even with the SDK.
In Q1, they will be looking to add the ability to support customisation however they have clarified that this is likely to still require custom development with their API to effectively roll your own widgets for the product page, using their SDK and/or collection of Web Components along with custom code.