Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
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It's quite possible that the Flickr guy didn't use shims. It's also quite likely that the lever clamps are only contacting at a few points, risking both rotating under normal force and possible damage to the handlebar.
Shims are an option and one that can likely be trusted, after all they are used for stem to bar clamps and that connection is stressed far more than the brake lever connection. Cross levers are often shimmed as well to fit different bars. However, you might run into problems finding a suitable shim due to the curve of the handlebars. You may get lucky and be clamping to a straight section though.
I did some searching and seems no one makes a 22.2 to 23.8 shim. However, the shim thickness you'd need is 0.8mm or ~.032" which is a very common thickness for aluminum sheet. Aluminum is fairly flexible at that thickness too. You could likely hand-form a shim out of a strip of material. I'd stick with aluminum though and nothing softer. Here's a link to get enough aluminum to make about 20 sets of shims for ~$7.50: http://www.mcmaster.com/#89015k27/=de6gte