Now that RubyMotion supports launching the app with Instruments, you can try this method: http://www.cardinalsolutions.com/cardinal/blog/mobile/2013/02/testing_a_location-a.html (written by my friend Rachel Parsons).
Ignore the xcode setup and skip down to where she talks about doing "automation" in Instruments.
To launch your app in instruments, simply run rake profile
.