Advice for small mosaic countertop in greenhouse?

I'm putting a 2' by 4' workbench in my new greenhouse, and I'll be setting a small stainless steel sink into one end. We've purchased a 2' by 4' piece of good quality 3/4" birch plywood for the countertop. I'd like to do mosaic over the wood.

We discussed using concrete backer board, but we realized we'd also have to build a wooden framework or backing to support the backer board, and we thought buying one piece of 3/4" wood might be simpler. (Cutting a hole for the sink through a piece of backer board and a piece of plywood sounded worse than just cutting a hole through one piece of wood.)

I've read that doing mosaic over wood is OK for indoors, and that I should seal the back and sides of the wood with an oil-based paint. Although it is a small countertop with a sink (I know, uh-oh, moisture) the sink won't be used as much as a kitchen or bathroom sink, and I don't expect the counter to be wet much at all. The bench will mostly be used for potting plants, so it'll be dirty a lot more than wet.

My husband is wondering if we should screw metal lath to the wood, and trowel mortar into the lath, and then let it dry smooth. He's thinking this would strengthen the countertop and also provide better adhesion for the mosaic. We had a tile guy put tile over our formica kitchen countertops once, and that's what he did.

Do you folks think a layer of lath/mortar is necessary? Do you have any advice (at all) about how you'd tackle this? Am I headed for disaster? Thanks so much for any thoughts, advice, warnings, or bonks on the head. :-)
Sheri