Not going to work. If you're going to do a mosaic like this, it needs to be a vertical application in a protected area where the freeze/thaw cycle won't bust it apart. Or, it needs to be applied directly to a well reinforced concrete slab individually or on mesh with traditional mortar and use grout between the tiles. And then protected against freeze/thaw by being under an overhang like a porch.

When you see exterior mosaics done, they are generally done in a climate without freeze/thaw.

if it's sealed with an epoxy and there is an internal weep system preventing moisture from building up under the mosaic, how can it freeze. This is something I will be doing, so it's a matter how I can achieve this to prevent the damage. does anyone else (experienced) out there have any suggestions?

You might try resin from a marine supply, but cement board is porous, and so is concrete. It will absorb moisture straight out of the air, much less a surface in contact with the ground. You're not going to stop that from happening without encasing it in plastic 100% and avoiding ground contact. Even then, the resin won't last forever before becoming UV damaged and brittle and allowing moisture inside. Your best strategy is to choose a different location to display your art.