Wood – safely affix tile to a panel of plywood

Here are some good points against using plywood, the source is talking about tiling a floor but most of the points are valid for walls as well.

In my opinion it's better to go the "overkill" route, then have to redo the job in the future because something didn't turn out right.

The cost difference between plywood and cement board is negligible, so your not going to save any money using plywood (unless you already have it).

Do the job right and use cement board, when you don't have any problems and you can enjoy the tile for years to come you will never regret the decision.

Wire lath has been used as a base for plaster walls and stucco for decades.

It is made from an expanded mesh of metal. You should be able to:

1. cut a piece that is the size of your mosaic
2. check to make sure that the lath + thinset + mosaic will be flush with the remainder of your tiles
3. attach it to your backerboard (or whatever the substrate is for the rest of the tile) with screws or staples
4. cover the mesh completely with thinset, making sure it fills the gaps under the mesh and adheres to the backing material
5. place your mosaic on the thinset.

The wire lath should be available at tile stores and possibly home centers. See, e.g. this