Lightb, I don’t think Abba was asking if the WWFoundation allows the notes when he asked if it’s permitted.
AbbaS, I have heard people ask whether pushing notes into the crevices might be assur, not because you’re undermining stability of the wall, but because you may inadvertently approach an area that has Kedusha. I don’t know the answer; I’d be curious if anyone else has heard anything like that. For myself I didn’t feel the need to put my bakashos down on paper.