Actually, the trail days have been used(this year anyway) for clean up and repair/replacement of existing structures in preparation for events(Poker Run/TT).
The only new stunts that I know of over the last couple of years have come by way of an individual(Reid) taking the time and initiative to build something on their own. The design is a result of the individual's artistic license and permit of the TC. I certainly appreciate the work that Reid has put in and hope he continues to contribute unique features in the future. I'm sure he just *loves* hearing how his efforts aren't good enough or don't mimic features from another trail.
I'd speculate that one would be permitted to work on a special project on a scheduled trail day, or could schedule an unofficial day, if help was required to install an approved feature.