Dan Mitchell's Music News points to some temporary Real streams of John Adams' The Dharma at Big Sur and Doctor Atomic - Easter Eve 1945 on the BBC Proms site. Available only through Sunday, August 29th.
In the introduction to Dharma, Tracy Silverman points out that the electric violin part is notated, not improvised.
Related posts: The Dharma at Big Sur, Easter Eve 1945
Update: Via ionarts, Andrew Clements of the Guardian reviews the recent John Adams Proms concert and makes the following observation about the electric violin part:
The instrument does not make a very varied or beguiling sound; it's more effective in the propulsive second movement of Adams's 25-minute piece than in the first, where it has to spin a long melodic line over a murmuring orchestral backdrop. But in the end only the energy of the music was memorable, never the musical ideas themselves.
Having only heard the work twice via the recent stream, I nonetheless concur. In comparison, Adams' Violin Concerto has both the energy and musical interest.