Yes, that's exactly what I want, and it's only for two slides that I need it.
To be sure that the back button is hidden:
For On Enter of the slide, use the simple action 'Hide Slide8Back'
For the Rollover event (On Rollover in the Actions Tab of the Properties panel of the slidelet), use this action:
Show Slide8Back
Maybe it is better not to use a Simple action here, because that will release the play head, but a one-line standard advanced action will do the trick.
Something you didn't tell: is there another object pausing the slide, like a Next button? I'm asking this because the pause for the Back button will not be active as long as that button is not visible. If there is no other interactive object pausing the slide, and the user doesn't rollover the slidelet area in time, slide will be finished.