Chain slap is a fact of life in cyclocross that I solve by wrapping an old tube around the chainstay as a protector. It also kills almost all of the noise.

As far as dropped chains are concerned, I haven't dropped a chain on my 'cross bike since I switched to a single front chainring and added a Paul chain keeper. A single-speed specific chainring is highly recommended but not completely necessary.

I use standard Shimano road/MTB rear derailers.