Now I'm thinking there may be a better way do reduce the (effective) input voltage. Since it is AC to begin with, perhaps a TRIAC with appropriate bias could be used to chop it down in front of the bridge rectifier, getting things into a safer 10-12V range perhaps.

???

Though I suppose if I'm at all concerned, I should probably just remove the entire front-end and instead drop in one the USB chargers discussed earlier. It will take the 24-28VAC input and produce 5.1VDC regulated output with no worries, and about the same amount of space taken up inside the box.