a. The hitting surface of the racket shall be flat and consist of a pattern of crossed strings connected to a frame and alternately interlaced or bonded where they cross; and the stringing pattern shall be generally uniform, and in particular not less dense in the center than in any other area. The racket shall be designed and strung such that the playing characteristics are identical on both faces. The strings shall be free of attached objects and protrusions other than those utilized solely and specifically to limit or prevent wear and tear or vibration, and which are reasonable in size and placement for such purposes.
No mention of wax or silicone, though they would appear to fall under the exception.
That may be a slippery slope, however.
They are not objects in the first place, else stencils, which are not even used to prevent wear and tear or vibration, would be illegal.