I don't think I've ever actually seen the Canon flash bracket in use before!
FWIW, I don't think it positions the light usefully - the intent of the bracket is for portrait orientation shots, to get the flash above the axis like it is on the hotshoe in landscape orientation.
For a relatively inexpensive macro setup (more so since you already have an OC cord), I've used the Manfrotto 233B Flash Bracket with Giottos MH1004 mini ball head. With that, you can get the flash head right out over the front of the lens (or hood), pointing down. That gives you a more natural illumination (we expect lighting from above), and the proximity to the subject means softer light. The setup is quite stable.
My even more stable (and much more expensive) setup for a single Speedlite is the RRS B91-B bracket with the FA-QREX2 extension. I use that mainly with my 600 II and a Better Beamer, for birds.
If you're considering greater than 1:1 (as you implied), that probably means the MP-E 65mm. In that case, I'd really recommend the MT-24EX. With the Macrolite Adapter 67C, you can mount the twin light on your 100L. Alternatively, I've found that a pair of Wimberley F-2 brackets allows much more flexibility in positioning the light where you want it. The MT-24EX is also a wireless (optical) master - you can have it on the hotshoe to light the subject, and use your speed light as an off-camera slave to light the background.
