I'm looking for some quick release brackets that would hold two pipes together at right-angles to each other. They also be quick release on at least one axis. I've drawn something quickly in Sketchup to try and show what I mean.

I've managed to find everything else I need from a combination of DIY hardware shops and bike/BMX suppliers. It's just little bits like this that have still got me flummoxed.

Drum racks use pieces like that, though you'd be limited to pipe diameters that are commonly found on drum racks, and you'd be paying the musical instrument price premium. Some of them clamp in such a way that they're adjustable to hold a variety of diameters, but they're all going to be in the 0.25" to 1.5" range found on most drum racks.

Theater folk use these.

http://www.starlight-online.com/clamps/

I haven't seen anything with a quick-release, but you might be able to adapt a non-quick-release fitting with a seat post quick-release.

Outside a bike shop, the only other thing I could think of is a speed-rail accessory, but those aren't quick-release, either.

What material do you need? Does this have to be strong?

Have a look for bicycle flashlight mounts, maybe. They're weak, but one or two might have the required shape.
http://www.dealextreme.com/c/bike-lights-electronics-16075

http://www.8020.net/interactive-catalog.html

See page 547. This is for a 1" dia tube.

Also there are these.

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