Hi Dan,
On all of my rifles except one, I have used Permalyn sealer. I find a few coats do seem to make the wood slightly more durable, but they have to be absolutely dry before the second coat, and likewise before the third. I believe Minwax also makes a wood hardener, though I have never tried it, so I can't report any results.
However, I have never tried using Permalyn sealer after the final finish was applied, so I also can't report on how that might work. I would think that you would need to strip the stock down to bare wood, just as if doing a complete refinish to be sure of any success. A test might be for you to try and steam out a dent that you have put in it, and see if it comes out. If it does, then the Permalyn might work over the finish you already have on it... If water can penetrate, sealer should be able to as well.
Beautiful finish on that rifle, BTW.
Matt