Your project has me intrigued! I mentioned the servo approach because I seemed to recall a discussion on alt.coffee where Andy Schecter actually turned up a good valve for that purpose for a reasonable price. I couldn't look it up before, but now that my son is in bed I gave it a shot, but can't find it.
But I did turn up this discussion, which uses a different approach, but maybe one you could use. Do I recall that you are comfortable with electronics? If so maybe you could even make the scavenged stepper motor approach that is mentioned later in the thread work. I know that people build small
CRC CNC routers to machine model plane parts with those things all of the time, so I imagine it would not be too complicated for someone who knows what they are doing. But the OP's approach seemed ingenious to me, and may even be better.
http://groups.google.com/group/alt.coffee/browse_thread/thread/83ede07c36ec20b6/fb5cf6388d920169?lnk=gst&q=stepper#[hit "expand all" up top to see the whole discussion, that link is the result of a google search and only shows the posts containing the search terms. The original post is what really caught my attention]
That said, having read a little more now, I would guess the ideal solution would be a proportional valve and PID, but as some mentioned in that thread, those valves are not cheap! But there is probably less risk of failure if you buy a good one, and if this project has some budget room maybe that would be worth considering.
Please keep us updated, what a great project! I wish I could be there to see how you work it out.
Paul