Could there be a number, such that for certain interesting data, you can compress the data more than normally achiveable, by indexing into the number?

I feel this should not be possible, but I don't know how to proove it without a circular argument about entropy.

On the other hand, if you make a string of common byte sequences, a couple GB long, and distribute it with every PC... then you surely can achive hyper compression with some clever indexing (if you don't count the magic string to the compressed size, of course).