Most of the tile cement makers do a two part cement containing lots of latex and the like that's formulated specifically for tiling over plywood on floors etc.

They don't to my knowledge recommend tiling directly on to natural wood - presumably because it may move a great deal more than ply in the cross-grain direction.

Personal experience suggests that even the cements designed to tile over ply sail fairly close to the wind - despite using a recommended build (factory recommended cement over tanking over primer over sealer over waterproof ply) i've seen problems develop - especially cracking up of the grout as a result of more movement occurring than it can handle.

Silicone sounds like a decent possibility, but even in that case all is not always what it appears. Best to talk to the tech support guys at a reliable maker if it's more than a one off hobby job. Thoughts include:

It probably matters to use a good quality branded high solids content silicone (as opposed to one of the cheap box store generics) because they should be a lot tougher.

Silicone of a given type may bond better on some surfaces than others.

Most silicones exude acetic acid during the cure which could damage some surfaces - there are no/low acid silicones about for this reason.

Make sure that the bond line (layer of adhesive between the two surfaces) is pretty thick - a thin layer results in the adhesive having to handle too much shear which can lead to tearing over time. Maybe even consider some sort of spacer arrangement - beads of an appropriate size or the like...

I've bonded two pack lacquered birch plywood to glazed tile in a bathroom, and four years later it's still going strong....

ian