Micron's patent for SE/30 grey scale system:

http://www.google.com/patents/US5307083'>http://www.google.com/patents/US5307083

with circuit diagram for grey scale video board:

[attachment=0]US5307083-4.png[/attachment]

Or this website, which basically describes what you're tying to do (author's just using a different video card).

http://www.google.com/patents/US5307083

Hope you'll get it going!