A minor tweak to template details XML allowed us to include cached user input.
Supplying user id and hash (ID, Hash) as part of api-template requests adds Value attribute to text and image fields in the template xml. This attribute contains the last used value for that field, but not necessary with the current template.
It is very convenient to users to get their last input as they try different products. Consider including it into your custom implementation.
Our web-to-print extension for Magento will include this feature in the next release some time this week.