The values shown on the device are not exposed through the ConnctIQ API. If a developer had access to the algorithm to generate the data from the inputs that are available, they could always do that.
Unfortunately, I believe the TSS is proprietary. I could be wrong, I've never really looked into it. I do know that it is not difficult to implement something similar to TSS. I've written up data fields that show what Strava calls Suffer Score (for running and cycling activities with heart rate data) which give you an idea of how difficult a session was.
