There is already that kind of topout spring inside of the dampening cardridges of RXV and other Shiver forks. So if you use second topout spring for reducing the travel that spring should be really stiff so that it won't anymore 'bend'.
Normally all the travel reducements for enduro bike forks (when they use same forks for supermoto and iceracing) are done by using solid spacers - at least in this part of world;) What is good in solid spacer is that you can lock it to it's place with locknut and therefore it stays (no matter what) at the bottom of the damper rod. If you don't lock it (or you spring instead of it) it may move up and down during suspension travel and leave marks to damper rod and then the upper cardridge seal doesn't seal that well if there is marks/grooves at the rod surface.
Again I'm not sure if this will hapen in reality or not, but I didn't wan't to take any changes what comes to reliability. After all it is an adventure bike (at least that is my goal with this bike;) and therefore reliability of every mod which I do for my bike has to be at top level. That is one of the reasons why I didn't want to go for gas pressurised gardridges like Bitubo etc..
And I'm glad if I can help you guys with this mod since I have got so many good advices and inspirations from this site that I am only happy if I can 'pay some of that' back by sharing this fork mod.