Caspio does not have the sorting feature in the View setup rather it enables users to set the sort order in the DataPage wizard however you are right if you have the results fields in HTML block you can not choose them in the sort order dropdowns. The solution would be to add that field as a regular field to the results page then hide it with Java Script programming.
What is the the layout format of your results page? Tabular, Grid, List?
Depending on each format the Java Script could be different so I would suggest you to ask Caspio Professional Service department to develop the Java Script for your purpose.
By the way we add this to the new feature request list to be considered in future releases.