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I found 2 ways which might show promise.

VoiceWatch Lap Timer. http://download.cnet.com/VoiceWATCH/3000-2094_4-75110828.html You would be able to actuate the LAP button hands free- even put it in your pocket.  Just note first lap start time and add intervals to that start time As far as I can tell, it is not licensed in US.

The trouble with using touch screen is you have to find the pads without blinding yourself (no prob for sun sights.) There might be an app which allows the whole screen to be a an assignable button or allowing the earphone switch to actuate.  So far, I haven't found that app. Basically a lap timer (there's one on iphone could be bent to the purpose.  Only problem is getting the button easily. But the lap timer with memory and easy access buttons is what we need.  No need to look at timer. It records intervals.  You just look at the face when you're through with the sights, right? There are many apps which allow the earphone button to click the shutter in camera app, but I bet somebody's made an assignable switch app or even a bluetooth switch who's function could be user assigned.