I can confirm that there are users on this forum who mod this game.
In case you get no answer from them I can tell you that probably a starting point is the data.jet file in the Assets folder.
It's a ZIP archive so open it with 7-zip or Winrar and use the correct password provided here:http://aluigi.altervista.org/papers.htm#info
I too would like mod bloons for a little extra fun. I have edited values in the data.jet file but when I load up the game, the edited bloons or tower is either removed completely or is just a sprite. Please help.