It is possible some add-ons (like t-shirts) could get added to the Larian Vault as regular items.
When the pledge management system was implemented Larian didn't include the ability to add money for addons due the amount of work it would have taken compared to the expected demand. There were a few such requests, though, and Larian said they would reevaluate that decision, but it has been awhile and if they changed their mind on that they have not announced anything, AFAIK.