Btw… If you do… make it clear such an act of criminal violence/vandalism was motivated by your grievance otherwise it’s likely to just be considered random crime and no one will know how much you cared.

Not in favour of crime, just don’t want someone who cares strongly enough they wish to throw the first stone for their cause, to go unnoticed or ignored… that would make it pointless violence instead of violent protest.

My first suggestion is the classic “wrap the rock in a piece of paper with your message on it” followed closely by fold the note up and tape it to the rock with good tape like duct/cloth tape but not in a way that won’t make it possible to remove the note and read it.