Hi Magne,
I think you would need a truckload of microballoons to achieve the same mix consistency, and I wouldn't want to be anywhere near your shop when you were mixing it.
It's really intuitive with the sand mix, to achieve a consistency that you like. On my wing molds, which were about 550 sq. inches per panel, I used 1/2 gallon of sand, with about 10 oz. of epoxy, per panel. I mixed it all up with gloved hands, and just pat handfulls of it into place, yielding about 1/2" thickness. (2 layers of 6 oz. cloth on the inside and outside) Using fine sand ("blasting sand" is what I used) gives much better results than coarse, heavy sand, which has a tendency to separate.
Lots of different ways to achieve thickness, including epoxy paste, from
http://www.freemansupply.com/ if you want to experiment.