How about crawling the top 1000 shaders on shadertoy in the categories popular and new and running it automated trough on your testfarm? That should give you a pretty good "ahead of time" warning if stuff's broken.
And reducing those to conformance tests can be done nearly automatically. Nearly every shader you find on shadertoy is pasted together from snippets of other shaders. In the case of http://glslsandbox.com/ they even have an explicit derivation feature for a shader. With a bit of text mangling and crawling you can build up a fairly good derivation graph for shaders, and bisect that graph till you automatically arrive at an offending snippet of code.