I'd be afraid that it would crack and fall apart because it's not a solid substrate and could "bend".

...unless you were going to attach your tessarae with silicone and NOT grout it. Then the tessarae could shift with the substrate.

But wicker is porous, too . . . .

I'd only do it fully understanding that it was temporary and I wouldn't cry if it fell apart.

I've seen where a guy took an old wicker chair, covered it with hardware cloth/wire mesh and then coated it with cement then covered it with mosaics. Turned out really neat. Pretty heavy, I assume but great for outdoors.

I was wondering this same thing. I have an old picnic basket with a woven top. I bought it at a yard sale for a dollar, thinking I was going to mosaic the lid. Once I got it home, I realized what I was getting into :) Maybe I could try some wire mesh first.