-- Last Updated: Oct-09-12 10:38 PM EST --
Might even exist for racing, as I recall they can be found in some pack canoes. But in both of these cases the assumption is that the paddler is going to be fairly unlikely to be throwing deep braces or rolls for the hell of it, or playing in rock gardens.... places where a slider seat would not be helpful, could be a real problem.
If you are talking about the ability to move the seat between fixed positions, that all happens from the top as you will be doing. (oops, I may be wrong there. See next reply.)
As to using the Tempest - have you told them what boat you have? Sometimes those rules, at least if you are talking about outside use, are there because they don't want people bringing inappropriately outfitted rec boats into a sea kayaking class. They may be fine with your bringing in a boat like the Tempest as long as they have a couple of minutes to make sure all the parts are properly attached, like perimeter line, before the class.
Pool sessions are another matter - between pool rules and space it is more usual than not to be restricted to the boats that are already provided. But outside classes are usually more about the setup of the boat.