Honestly, I'd try to find another piece:
1) Cedar releases aromatic compounds into your tank, even if it doesn't smell like cedar anymore.
2) You'll have a constant rotting process going on in your tank. Not only does that increase the pollutants in your tank, it might also affect the fish (I wouldn't want to live in a house with mold)
3) You'll have a hard time sinking that puppy ...
It might work short term, but chances are you'll run into trouble sooner rather than later.
I always collect my driftwood from rivers when I go canoeing. Marine driftwood from a beach would work as well. Just two ideas for a fun weekend activity combined with driftwood harvest (once it gets warmer)
HarleyK