I need a set of MAC/Ethernet addresses that I do not have to worry about collisions with, analogous to
192.168.x.x: addresses that will NEVER be assigned to real hardware.
You want to set the universal/local bit in the MAC address to 1. This is the second-least significant bit of the first byte of the address.
All MAC addresses physically assigned to real hardware will have this bit set to 0. Locally assigned addresses are supposed to have this bit set to 1 (though this requirement is often ignored).
Thus, any address beginning with x2, x6, xA, or xE will work.