Interesting, I'd like to understand what you mean by "independent of DAW".
What I think you mean is that you just record yourself playing notes, no metronome and no "loops" structured in a DAW timeline, and your synths "change over time" in that you also modulate or change their parameters as you are playing (like this talented gentleman:
http://www.youtube.com/watch?v=2xsHN3NTyk0). In short, it's not a super structured song, just you feeling it out. Am I close?
That's exactly what I did in my new song, the poem with improv (link in sig). I have 5 different instruments, and I just went through and played whatever I felt on each of them, layering the song as I go. I ended up doing three different versions, then splicing the parts together.
Anyways, I feel like if you were to add precussion to an ambient song, but not "bind" yourself to a tempo or time signature, you could just do the exact same thing. Just listen to the song and record in yourself playing whatever whenever on the drum rack. And then maybe some heavy processing to blend it all in.