There is a core Splunk module called "EnablePreview" that you can use today. It's a bit clunky and works differently than other modules, but it works, and I haven't yet gotten around to creating a better way.
The weird thing about it, is that you have to put the EnablePreview module below the "dispatch point". If you remember your reading from the "Introduction to the Advanced XML" page in Sideview Utils, there's a certain point in the hierarchy below which the framework will have automatically dispatched the search. As long as the EnablePreview module is at that point or below, you can use it to turn preview on or off as you like.
To have it just be hidden and turn preview OFF,
<module name="EnablePreview">
<param name="enable">False</param>
<param name="display">False</param>
</module>
and if for some reason you want it to be a visible checkbox that users can click while the job is running, that will indeed turn preview on/off right while the job is running.
<module name="EnablePreview">
<param name="enable">False</param>
</module>
