I see, there are so much demos I forgot about this one, sorry. Well this demo shows me another way to do it, but it brings back globally to my first solution, which is editing one (or more) FlxText. Something I should add is that I need to sometimes break the flow of the box when it's asked or when the textbox is full. (I possbily have a control system in the works to embed stuff like querying entities' names or pictures/sprites) That's why I wanted to run my own version of such system, I already have one working in C++ based on the second solution. (Beware, it's quite hacky, I have its mechanisms in my mind, I'll just redo it from scratch for a cleaner version).
Lycan's system, even if half-implemented, is related to my second idea : having a list of sprites used as letters. I have to admit this is the solution I'm preferring as it offers me the ability to mess with the letters independently of the whole line. I'll guess I'll use option 2. Thanks for the suggestions. I just now wonder about the performance impact of FlxText against using atlases, but I do think I should take a look more at Flixel's sources, shouldn't I? Hehe.
At least, thanks for replying to me. have a nice day and good luck with developing this library!