Geoff, the weight of the mouseballs will create a sound problem. I haven't tried it, but I would guess that secretly dropping a mouseball into a cup will make much more noise than dropping the equivalent rubber ball.
Also, while they may still be great if you only want to use non-magnetic balls, a problem arises once you start thinking about the magnetic balls for chop cup routines. Specifically, a mouse ball weighs in at FOUR times the equivalent rubber ball. Given you won't have a magnet in the mouseball, the magnet in the chop cup would have to be around EIGHT to TEN times more powerful. So, mouseball cored balls would be virtually useless for any existing chop cup.
Jim, I don't know if these are the dice that you were thinking of, but they do make dice erasers that might work. There is very little quality control with these dice and they will range in size from 3/4" to 1", and aren't quite cubes. If anyone wants to buy some of these, have a look at Northeast Imports
. The dice are available in orange, blue, pink and a few other colours.
Although I haven't used them, Mike Roger's baseballs had the right idea. They had the right size and feel, but I don't know what the weight was like. The problem was that if you didn't want to use little baseballs, they did not come in the right colors combinations(ie, stitching color matching ball cover). Also, the balls didn't come in enough colors (like Geoff said, you need at least three). Finally, the chop balls didn't have the right configuration (two half-sized magnets to maintain a single external pole opposite to the magnet in the cup).
Bye for now