Similar to "Label Colors for Keys" suggestion, with the added functionality of triggering expressions.
Create a keyframe
Right-Mouse Click the keyframe and select Scripted Key
Write parameter specific script in pop up script panel
Scripted key is red with an "E" or "S" on it and triggers expression when playback head hits it.
You could also select script range, and bar is created that indicate the start and end of the script.
No expression controls need.
Michael Szalapski commented
I kinda like this idea. Although, you can already have expressions set to trigger on layer markers and composition markers, so you can already do quite a bit of this.