If you leave it "as is", I'd first stain and seal (polyurethane?) the woodwork, to include the bottom edge of the plinth block.

When installing, hold the plinth block off the granite by an eighth inch or so, and silicon the gap after all has been installed.

Alternatively, you could set the granite, then run a bead of silicon on the granite along the footprint of the block, then set the block into the silicon. Remove the squeezeout for a clean look.

If you decide to cover the plinth with the glass tile, I wouldn't. I'd substitute a square of cement board in place of the wood plinth and run the glass over the cement board. If you need additional thickness, use two squares of cement board, one stacked on top of the other.

Do think through the detail on top of the glass-tiled plinth, and how you'll detail the fluted column where it runs into the top of the plinth block.

Also, visualize how the fluted column will look sitting on top of the glass mosaic tile. While it might look nice to carry the glass mosaic in a continuing band around the room, will the stained fluted column look okay as it floats on top of the glass?

A mock-up is definitely required, and you have a good start on that already with what you've done.

FWIW, I use painted wood backsplashes quite often, and stained wood backsplashes occasionally. Never a problem.

Mongo

THANK YOU Mongo. I was hoping you'd chime in on this :)
After I posted this question, I looked back in my archive of pictures and found quite a few granite vanities with wood columns (and cabinets) sitting directly on top of them. In fact, the inspiration for this design is like that. OK, we'll not tile the "plinth" (I learned a new word today) :) DH has been looking at new construction and came to basically the same conclusion as your other suggestions. Thanks for the details!
Monica