Hi Xsue,
I don't agree that all the splashes go on to the lids, I have had one for 15 years and the splash back at the back gets splashed with fat etc, I have tiles. Tongue and grove is a personal choice, I haven't seen it before. The thing I would be worried about apart from the splashes is the heat the wood would expand and contract and lead to splitting. Maybe if it is a new type of aga that you can control the heat this wouldn't be a problem but it can be quite hot around the aga.