In an effort to get Sarah's FS rig ready for our Tucson trip, I threw together the direct mount front derailleur setup today. Learned a few things to do differently next time, and took a few (cruddy) pictures. I think it'll work really well, but I can make it look a bit nicer.
-It's dead easy, from the framebuilder's perspective, anyway. You can see the mounting specs here. Thanks to Pete for the scanning work. Total time to build and install the mount was probably only 15 minutes. It would be relatively easy to have some generic mounts made that could be mitered and fitted up much more quickly, too.
-Total weight (derailleur) is 140g. The mount I made is 16g. About the same weight as an old-school M750 clamp-on unit, but with a lot of nice seat tube location flexibility built it. Good for weird configurations. Pointless for conventional setups.
-You can adjust the height of the cage by about a total of 5mm, as the derailleur has a little slot on it. You could easily make a mount for the frame with multiple mounting points to adjust height as well.
-The derailleur comes with a built in cable stop and bottom-pull arm. I cut them both off, which took about 10g off, but more importantly removed a bunch of annoying crap that was going to get in the way.
-I didn't bother to make a slotted mount (see the link to Shimano tech docs to see what it's supposed to look like) and am just depending on using the square edge of the mount set up tightly against the ridge on the derailleur. I think it'll work well, if it doesn't, I'll redo the mount.
-The bolt protrudes through the mount and is visible, which you can see if you squint. I could have set this up so that the bolt isn't visible (and routed it through the part of the mount that connects to the frame) but I wanted to be able to add a nut to the back side if the mount gets stripped for some reason.