I know your post relates to making them lighter but i just thought id share this with you for if you ever intend on changing frames in future.
Just a little tip from my experience of running middleburns and from working in the cycleshop trade.
Been riding now since 2003 and i have always sworn by middleburns (recently upgraded to a heatsink cloud9 bash which is very good!). Great cranks for all types of riding due to them having interchangeable chain rings and spiders etc...
The only minors i have come across mine due to trials parts changing and technology evolving is the clearance of middleburn arms which do not exactly have a lot of clearance... Reason being on my 06 gu, piranha and my ozonys my cranks would catch on the brake cylinders every so often which involved having to grind my arms which was very scary to do. I even had ran a 128mm bottom bracket to try and get as much clearance as possible!
With middleburn been around for many years and there rs7 cranks only offering 28mm clearance plus they are sort of the only cranks you can get to avoid screw on freewheel/sprocket on modern cranks. But modern cranks all seem to be running on 35mm clearance on the arms which seems better. If middleburn did this i would stick by them for the rest of my life as i am now debating to change cranks to more modern/screw on. But i have never ran anything like this before its basically new to me.
I think the middleburns would now only seem to suit for the streety bikes such as inspireds etc...
Sorry if ive clogged your thread with this just felt the need to share this with you.
Regards Andy