MikeY,
It's quite likely that 'Foggy Dew ' simply arose as something that made sense to people who had never heard of a 'bugaboo' or 'bogul maroo'. Though the latter survived in a few versions it does come to us from a 17thc broadside. Similar words of course are still in common use, 'bogeyman' for instance.

Michael,
Unless you know otherwise there is absolutely no evidence even to suggest that 'foggy dew' in this instance is anything but literal. However, of course it is fun to speculate. But that's all it is, fun.