"Guest judging" is in intriguing suggestion, Ultima15, and one HERC can talk about. :-)
The HERC judges are a bit stuck for timing for the Costume Contests - we have a set start time, and need to stick to it to be fair to everyone. Lately, yes, that has meant starting judging during the 'evening' hours, but if we wait till the sun rose again before we started, then it would invariably be dark by the time we take the winner/honorable mention screenshots, which would mean asking the winner/honorable mentions to hang around another extra 10 - 15 minutes in order to get theirs screenshots taken during daylight again.
Speaking for myself, I do understand you're point about seeing costume colors in daylight. Generally by the time we're done judging, the sun is rising again, so most of the time we do get to see the costumes with both night and day lighting, before we announce the judges results.