When you say "it doesn't work", do you mean nothing shows up or what?
You shouldn't have to invalidate more often than the number of times you are manipulating opengl outside of jME's control.
oki then I guess that invalidating was the right way to solve the problem. Yeah if I don't invalidate the state the next TextureState append() will not setup the font Texture. Oh well it's a bit hard to describe.
The question is what defines outside of jme's control, but I guess i'll learn all the inner workings of jme at somepoint 
O well maby someone can take a glance on the code if you want to include it in jme. I took the freedom to use the package com.jmex.bitmaptext just in case. I do think lots of ppl might find it useful.