My mounting of a lowrider rack is a long (but interesting) story. The small front rack (from Nashbar) that I adapted for supporting my handlebar bag lower that it would sit mounted on the bars (for better front end stability) mounted quite successfully from the brake posts (and the fork top). After doing some research and looking around at front lowrider racks, I found some that were meant to mount with clamps to suspension forks. They looked to me to be adaptable to mount to the brake posts just like my small rack. I found one of these racks at MEC (Mountain Equip. Coop) for a very good price. I made new upper brackets from aluminum to allow the rack to sit a little lower. I also made aluminum brackets to mount to the lower dropout braze ons. Flipping the rack hoop up to mount to the small rack was pure serendipity, I hadn't planned it. It works very well and the whole unit is very sturdy.
Without the brake post mounting points (such as on a disc-braked bike), I'm not sure what I would recommend. I certainly would not suggest trying to clamp anything to the fork legs, anywhere.