Updates
The float worked great on Halloween!
Core Construction
The running gear was fine, even over speedbumps, and the float platform held up great. The only issue there was that the yoke or tongue or shaft (?) which was used to pull the thing was probably a little short. There didn't seem to be a non-awkward way for two (admittedly over 6-foot) people to pull it. Next time, I'll investigate lengthening it, if that can be done without sacrificing too much. Additionally, the wheels do rub if you attempt too tight of a turn, which is something to work on next year as well.
Fog
The fog machine also worked fine. In the run-up, I found that the cheap 5V water pump I had originally used wasn't really sufficient to move colder mineral oil, which can get kind of viscous. I ended up replacing it with a 12V pump, which necessitated finding a way to provide 12V of power. That's just 8 AA batteries though, and a little case that connects 8 batteries isn't too hard to get.
On the day of, I panicked a little and installed a cheap 5V usb-powered "personal" fan in the vent tube just beyond the chiller in order to create an air current that would pull the fog through the chiller and into the distribution tube under the float. This did sort of work, in that it pulled the fog through the chiller, although my installation did turn out to be a little leaky - next time maybe use more tape and, if possible, some kind of fog machine to blow visible fog through the thing so that I can more easily detect leaks.
One thing that didn't occur to me while planning and constructing the thing, but that seemed head-slappingly obvious after walking behind it for 10 minutes is that it's nigh-impossible to get fog to stay under and around a float that's moving faster than a snail's pace.
A moving float, especially one that's boxy, and even one that's moving slowly, pushes a lot of air when it moves, creating high pressure in front and low pressure behind, and the result is that any fog released underneath or to the sides is immediately sucked behind the float and into the turbulence in its wake. This means even your best efforts to chill the fog in order to keep it near the ground may be in vain, as by moving the thing down the street, you're doing even more to pull the fog up and into the swirling eddy immediately behind it.
Other than the fog mostly swirling in the float's wake (which still looked cool), there were no issues. We bought 50 lbs of dry ice on Saturday morning and by Monday night we still had more than enough to fill the chiller, probably 30 lbs. I started with a pound of propane and 2 quarts of mineral oil, and there was plenty of dry ice and propane and oil left after an hour of running.
Sound
No issues with the sound system at all. I was a little worried that we'd get in trouble for being loud, but it seems like if people are into what you're doing, you can get away with a lot. The batteries lasted sufficiently long and the music sounded great. Still want to figure out how to smallen the apparatus, as it's a little bulkier than I'd like.