Would it be worth considering setting the speakers in the stone wall? We've done that; standard outdoor "bookshelf" (ie not in-wall) speakers, in a recess in the wall, with metal grille painted to match overriding color of the wall.
I can't find pics of mine, but...
[Link: goo.gl][Link: goo.gl]OR,
would Stealth Acoustic Stingrays be an option? "To offer a completely unique finish option, select the Image Wrap version to have an actual photograph of the speaker’s surroundings or even artwork laminated directly to the face of the speaker."
[Link: stealthacoustics.com]