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Carl have you taped over the viewfinder port on the 6D? Given that there is a prism breakdown of the light it suggests that it is passing through glass somewhere on its way to the sensor surface at an angle. Even if it is hitting the sensor coverglass at an angle (Canon stock coverglass is known to reflect as it is not multicoated) this might cause it. The smaller sensor of the 500D probably still has a coverglass over the sensor but less area overall. Is the viewfinder port covered over on the 500D? I had a light leak on my 450D earlier this year, only apparent on long exposures like your eight minute ones, turned out to be a coming in through a gap in the collimation ring and reflecting in the tube to the sensor - a bit of black tape stopped it.
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