Funnily enough, I came across this thread because I was looking to see if anyone had tried beyond 100mm travel on a 1990s hard tail. A while ago, I fitted my 1998 Stumpjumper (small) with a Rockshox Recon 100mm and thought it handled okay but was disappointed by how far the fork dipped into its travel. Good on small bumps, bad for everything else. A DIY volume spacer improved engagement and gave the fork more mid-stroke support, but it was not enough.
Then the other day, I realized I could remove all-travel spacers from the air spring and make the fork have 120mm of travel. I tried it for the fun of it and was not surprised at how slammed the bike looked. With that having been said, 120mm of travel seems to be the sweet spot for this Recon fork, because it feels 1000x better and extremely plush. Also the sag is enormous anyway (with the recommended PSI for my weight), coupled with my weight being over a short frame, so the fork generally sits low enough in its travel to where it's at about 90-100mm travel as I ride.
I genuinely want to keep it like this because it's a much more plush fork when extended to its fullest travel. The only thing I am concerned about is climbing (in)efficiency and potentially snapping the head tube.