I'm sure there's a much easier way than this, but if there doesn't turn out to be, maybe you could open a copy of bryce with the default sky and export it to a preset sky file, then exit and load your bryce scene and then import that preset sky file? Haven't tried it myself. Not sure what things are included with that export. Supposedly to reset sky & fog pallette settings you can just click on the memory dot in the upper lefthand corner. However that doesn't appear to reset things like the sun position for instance, so I don't know if that would work for you depending on what you might have changed.