I'm going to just leave this here because the abstract graphic explains what this team is doing. Why enrich every sample? Enrich one, use that as a "Boost" channel at a slightly higher level so that is the dominant ion selected for fragmentions. Violin! You've effectively phosphoenriched all of your multiplexed samples.
I will also go so far as to say that this works great in our hands here and I might consider a confirmatory study even.