The idea thing would be to measure the speed of light for higher energy forms of light, like x-rays and gamma rays. But I'm not aware of any experiments that do that.
Err... build a broad spectrum telescope and look at an unstable stellar entity?
That's an interesting idea. But the method one detects gamma rays or x-rays is very different than what one uses to detect light, so calibrating would be tough. And most unstable events take place over time, so this would be really tough. Look at for example a supernova- even the neutrino burst lasts on the order of tens of seconds. Telling whether the gamma rays arrived at just the right time or not would seem to be really tough. I'm not sure, would need to crunch the numbers. It certainly is an interesting idea.
Hmm, what about actively racing them? Same...
http://www.nature.com/news/2011/110922/full/news.2011.554.html
http://arxiv.org/abs/1109.4897v1
http://usersguidetotheuniverse.com/?p=2169
http://news.ycombinator.com/item?id=3027056
Perhaps the end of the era of the light cone and beginning of the era of the neutrino cone? I'd be curious to see your probability estimates for whether this theory pans out. Or other crackpot hypotheses to explain the results.