When I use the wood, I always either screwed it to a piece of slate or weigh it down with rocks, so to be honest I am not sure how long it will take.
I think that if you were to weigh it down at the bottom of the tank, the over pressure will help to get it waterlogged. There is much more pressure pushing down on the wood when it is 1.5 feet below water as opposed to a couple inches. The pressure will help the water penetrate into the pores of the wood ( I could possibly be totally full of it though..it just seems that that should be the case). I wouldn't worry about a rock changing the chemistry too much. There is not much limestone around where you live, mostly igneous and metamorphic rocks which won't alter the chemistry much over the time frame that you would have it in there (if at all).
I used playsand in a 55 gallon, the detritus is certainly easy to clean up, it is also VERY easy to see if you don't keep on top of it.