You very well can "jumpseat" on a many airlines up front. The problem isn't CASS, it's your name being on the MCL, and if your name does not appear on the MCL the airline must do additional vetting on you, as an individual. This is something most of the "high crust" airlines don't want to do. It also comes down how each individual airline's security program is written and what is approved by the TSA for that particular carrier.

99% of the time employees of the airline operating the flight (and it's codeshare partners, funny enough) are allowed to sit up front without additional verification, as written in the generic 12-5 security programs the TSA issues.

Additional things like the manifest and individual clearance come into play as well. CASS itself is nothing more than an electronic means of employment verification.

Just about any of the ad-hoc/supplemental cargo carriers will get it done for you and welcome you aboard with open arms.