Hi Peter,
Thank you for your interest in Syncfusion Products.
We regret to let you know that it is not possible to get the hidden column values through javascript. But this can be achieved using server side where the columns and visible columns are maintained as a collection.
Please refer a workaround sample from below which illustrates retrieving currently selected column and visible column values.
http://files.syncfusion.com/support/GGC.Web/7.1.0.30/F80387/main.htmPlease let me know if you have any concerns.
Thanks,
Rekha