Efficient methods for flagging profile submissions from a contact form
Your solution to use a hidden field seems reasonable. I would suggest though, instead, you create a custom field (on the same dataset) called Triaged, with a default of 'No' (or null).

Don't even bother including it on the Profile.

Then when the contact us profile is manually processed, the staff member checks the Triaged field (or sets it to Yes) on the backend.

No jquery needed (there still are some people that don't have javascript enabled). Let the system do the most it can for you.  

As @petednz points out, Drupal users have a way to handle this using Webforms. If you are ambitious, you could build a form on the WordPress side and on its submission run a bit of PHP code to call the CiviCRM API to do what you need it to.  But that's way more technical than most people want.