Thanks, Gordon and Matt. The tubing is 3/8". So I could source more pretty easily to get it to the bottom.
However, my question is would I loose much efficiency as far as getting O2 into solution by removing the stone from the equation and pushing the O2 down the liquid dip tube in the keg? That way, I could just throw a liquid disconnect on the tubing in place of the stone, open the valve a bit, and pop it on the liquid post on the keg. O2 goes to the bottom and bubbles up. Bigger bubbles, yes. But must easier to sanitize since it's sanitized anyway. This is how I carbonate my kegs, though obviously not with O2.
I guess there's only one way to find out.