Dog-proofing hardwood floor
You all were correct. Partly to satisfy my own curiosity, I did some tests to see if hardwood could be "urine-proofed". I took some planks of hardwood and tried the following methods to seal them:
Titebond T&G glue applied inside the groove
Titbond T&G glue applied to top of tongue
Silicone sealant applied inside the groove
Silicone sealant applied to top of tongue
In all cases I used a good amount - enough that some excess squeezed out when the boards were tightly pressed together and had to be wiped off. My sealed planks were allowed to sit overnight, then tested by carefully pouring a small puddle of water on top of each set. I then checked the boards every few minutes to see how well they resisted the water.
Here's what I found. The glue didn't help much. Water stayed pooled for a short time on the surface, but quickly made it's way around and through the adhesive, penetrating all the way down to the bottom of the planks within 10-15 minutes.
The silicone fared better. Water stayed pooled on the surface for 30 minutes, at first giving me hope that a true watertight seal had been created. But after another 10-15 minutes went by, I noticed the water was slowly disappearing. It didn't seep down and pool under the planks, like it did when glue was used as the sealant. Instead, the water simply absorbed into the unfinished wood of the tongue & groove joints themselves, right at the top layer. All along the joints, the top 1-2mm of wood became waterlogged. So the silicone did it's job, keeping the water from penetrating down to the subfloor, but it couldn't protect the the topmost layer of wood.
So there's just no way to effectively waterproof (or urine-proof) wood. At best, you can use silicone to buy yourself about 30 minutes of real protection, but beyond that the water (or urine) will start absorbing into the top 1-2 layers of wood (at the joints). Left long enough I'm sure it would get completely saturated.
Therefore, hardwood is out. I'm going to go with a wood-look vinyl product like Konecto or Flexitec.