It would be a waste of time here, lets face it our roof tiles come with feet hating knobbly bits. If they are flat, free of flaws (life threatening cracks) then there would be no reason why they couldn't be used. You have identified that they are porous but then so are the meriad of stone tiles available, and other than the shower area probably no great reason to seal them. Sounds like an affordable plan to recycle the old roof.