Yes, dogs can safely eat plain rice cakes in small quantities as a snack.
Make sure to break them up so your dog does not choke on them and only give your dog the plain variety of rice cake.
Your dog cannot have any sort of rice cake that has extra seasoning or toppings because some of those ingredients are bad for your dogs health.
Some can even be toxic such as garlic, onions or toppings with too much salt.
Therefore yes, you can give your dog rice cakes as long as they are of the plain variety and in small quantities. Like anything else in life too much of one thing can be bad for their health.