I think Mingus pointed the way as much as anyone for free music, whether his ideas were written or not. Especially the quartet with Dolphy and Curson. My old Quincy neighbor Ra Kalam Bob Moses used to sit in with that group as a teenager!
People like to use the criticisms Mingus wrote of Pharoah Sanders as a dismissal of free playing. Firstly, I don't think Mingus would think much of conservative straight ahead players now, secondly, his work is actually more important to the history of free music than Pharoah, as much as I love Pharoahs' Impulse LPs, thirdly, it doesn't get much more free and open than this:
Yeah the Dolohy and Curson quartet is really seminal. This record is a new one for me though, but I'm enjoying it so far. I definitely do agree Mingus is a very important contributor to free music. His music was my first introduction to free playing and other avant-garde compositional approaches. And much like Sun Ra, Mingus is still very much ahead of us all.