Use 200 bit's to encode the message except the last bit which is determined if the message had 100 1's or 101 1's. QED knowing the number of 1's buy's you at least one bit of information.

Edit: You can do better than this by counting the number of 201 bit messages with 100 0's which is well below 2^201 and then encoding which of those corresponds to the original message. Aka if you had an 8 bit message with one zero 11111101 you have a total of 8 options which means you can encode it as 3 bits.