I havent done it myself but remember reading about it in the American magazine 'Wooden Boat'. The yanks sometimes call it wedge seam construction.
The article was about new construction where the builder had a special sawblade set up for the splines.
I would be a bit wary of doing it on a boat that had been in saltwater as adhesion problems could be a risk. If done properly a very watertight and fair hull can result.
I have a 44ft Mahogany sloop originally built in 1963 with a splined hull. The hull was relanked underwater four years ago, all splines removed(above and below w/l)and replaced. She is currently in the Med where, so far, the sun has been unable to move the splines so they must have been done properly.
Snag. I didnt do it myself, it was done by the boat's original builders R.J.Prior of Burnham on Crouch.
Try phoning Robin Prior, he may be able to advise, even if he does try to persuade you to take your boat to him.
I had my wooden S&S splined Spring of 97. The hull is edge glued inch thick Utile. The job was done professionally and every attempt was made to dry the hull out and season the wood for the splines. Despite this I now have at least 6 seams opening up again. The glue used was epoxy and the seams were routed out with a wedge shaped cutter that was purpose built. The only advice I can offer is do the best job possible under cover and paint her a light colour afterwards. I'm not convinced epoxy was the best solution, but even now I am at a loss as to what else to use and how I would effect a repair without starting all over again. For me sheathing was not an option. I'd sooner pump than polish.