Hi Peter,
I haven't put the finger mouldings on the back, I have PAS so didn't feel the need. It would be possible but very labour intensive without some form of jig to get even depth and spacing. I note your concern about the potential for it to be slippery but many of the aftermarket Motalita stuff is totally smooth as well plus this rim is a lot thicker which gives far greater purchase.
In the event of an accident I suspect the wheel will deform in a very similar fashion as the wood is not very thick and the spokes should deform in exactly the same way. If however the wheel rim does prove to be more sturdy all it will do is spread the impact load over the whole chest rather than a direct hit on the sternum from the centre boss. Don't watch this space as I don't have any intention of testing my beliefs.