I have always written a lot, but I never did it as religiously as morning pages. I wrote a wiki in 2004 where I keep all my thoughts/notes -- it's about 1000 wiki pages now (probably 2000 without deletions). Before that I kept a diary but it wasn't really for "ideas", more "complaints" :)
Right now I feel like I have enough ideas and it's about execution... but I do think there is something valid to the approach.
It's also similar to what happens when you write down your dreams first thing in the morning (or even during the night if you wake up after them). I tried this when I was 15. My mind was blown.
I started remembering 7 or 8 dreams a night. I would go to sleep for 50 minutes and it would feel like I experienced 8 hours of time in my dream. It was really disorienting (and interesting). I don't remember but I think it took about 2 weeks of writing before that happened.
So this is another example of writing changing your brain. Your brain wants an audience (or more accurately, your subconscious does, and the subconscious is where ideas are generated). If you write down your dreams, you'll start having/remembering more of them. If you write down more ideas, your brain will generate more of them and follow more associations.
Deliberately plunging my brain back into the places and situations I'd been in recreated the super-stimulated "traveling brain" state. I think the act of reliving and reinforcing the memories made them even more vivid now several years later than they otherwise would have been.
I believe the key aspects of the process were both the deliberate memory churning and the typing--the physical transfer of the brain activity. Maybe my ~80 WPM typing speed happened to be a good match/governor for my recall rate, too.
Now this wouldn't be at all practical on a day-to-day basis since day-to-day life details are so repetitive, but I could for sure see the benefits of picking some out of the ordinary event or encounter and drilling back through it to discover what else your brain has to offer. I can also see the potential in trying it for a known future event for planning purposes.
I don't think I could write in a notebook anymore -- my handwriting has atrophied and my hand would cramp up... :(
Once I got into the habit of doing them, I saw three other benefits: 1) I no longer needed stupid to-do lists, 2) Because I did them right when I woke up, before I was conscious enough to be self-conscious, I was brutally honest with myself, and got rid of my obnoxious depression, and 3) The daily practice really improved my writing.
Journaling really is a better form of meditation. I wish I'd gotten into the habit sooner.