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According to this article the Second bit of the most significant Byte indicates if the MAC address is Locally or Universally administered. (I think locally is better on for the public facing so you do not collide with other mac addresses). When generating MAC addresses for macvlan, this bit should be set to '1', indicating it is Locally administered. As such translated to a mac address any of these numbers are locally managed and you will not need to worry about colliding with the larger Internet,
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Not sure I can use comments here... need to investigate if it causes issues.
Generate mac address same way lxc does - http://giantdorks.org/alain/how-to-generate-a-unique-mac-address/