Much thanks to Neil Deakin, who commented on my previous post with a pointer to the Gecko SDK. The Gecko SDK allows developers to compile XPIDL typelibs without having to fetch and build the Mozilla source tree. However, as usual there is no Mac version :P
I've zipped up my copy and put it here. Compiled from the 1.8 tree, on OSX 10.4.5.
Thanks to Alex Sirota for his helpful instructions for building a standalone SDK for OSX.
UPDATE: This evening, the surge of immense pain brought on by the masochistic activity known as "jogging" caused this thought to arise: "Oh shit. Are there any licensing issues with having some Mozilla binaries posted here?"