As I know, Mju have sensors for metring light placed between flash and lens, not TTL metering. So, if you cover those sensors you can "fool" flash in a way to give stronger output. If you do not cover sensors, but cover flash with diffusion material, you will get diffused and weaker flash output, depending how transparent material you use. I don't know if covering flash and sensors with same material will help, that is you will get diffused flash output, but will covering sensor with same material raise flash output enough to compensate covering flash, I dont know. Why don't take roll of film and play a little?
For Rickoh I don't know, but if it have non TTL sensors, you can use same principle. If you have manual for those cameras, there should be written if cameras have TTL or on body sensors for light/flash metering...
If there is TTL metering and you want to cover flash with diffuser, you could need to compensate with film speed, and john already told about that, there are DX stickers avaliable for "fooling" DX camera sensors.
Of course if camera use preflash to meter flash output (check camera's manuals), then you are lucky, no matter if camera have TTL or on body snesors, covering flash with duffuser will be automatically compensate, so you can just cover flash without thinking about else