The wild weather is exactly why I've never gone all-in on a costume. I can't wrap my head around planning for so many variables and being comfortable.
My 10k test run a couple of weeks ago was my first time ever running in costume. My thoughts on running in costume based on that, plus speculation and wishful thinking:
1. I'm willing to put up with a little bit of discomfort for the fun factor, especially for the shorter races.
2. The 5k and 10k should be done before sunrise, so if it is typical weather, the costume = warmth. For instance, I *really* liked having gloves on until things warmed up in my recent race. Race finish was in the upper 50's and I wasn't hot or uncomfortable afterward.
3. Similarly, the half will be mostly done before it gets very warm. (Assuming the usual start temp in the upper 40's / low 50's. )
4. I'll be dressed in layers and will adjust any underlayers per the temperature forecast. If it is cooler than typical, then the costume is bonus warmth.
5. If it looks to be one of the really warm races, then I'll wear just the moisture-wicking costume shirt with shorts and forego the costume fidelity
6. Atypical for me, I'll have extra luggage to accommodate possible weather changes
7. No way that I could wield a light saber or anything else for more than the 5k, so any props would need to be strapped to my hand/wrist/arm. And to have something hanging from a belt = chafing for the longer races.
8. See #1