Or it's too damn complicated.
Unfortunately, most of the skilled modders hardly visit this board due to them constantly working on their own projects. For the rest of us average modders, we don't have the skill to accomplish these tasks.
As for my input, this mod would be two parts.
First and most importantly, it needs it's own playertype. This playertype would be a fuel jet player, but the fuel does not recharge on it's own. The player is only able to jet while the FLUDD is in the players inventory.
The second part would be the FLUDD tool itself. Firing it would drain energy from the playertype. When not in use (ie, not the active tool yet is still in inventory), it remains on the player's back (Similar to GC's weapons).
I'll model it tomorrow when I get time. I'm not sure if I am able to script the weapon, though I could give it a shot. I could also attempt to do the playertype, but don't count on me finishing it.