Adding new field to session

[Opserty]

You can't change data in Hidden fields they are hidden... (the user has to check the HTML first to see they exist).

Unless you intercept the header requests and all that malarky. (Which is not something your average user can/would do)

Data is sent when the user submits a form...unless the form has thousands of elements passing the data twice won't have a noticeable impact on the performance if it has any at all that is.