I'm posting this here for suggestions/comments, the script is early alpha so it's just about usable. I have plans for it like tilemap position saving and a few other things but I haven't been able to find the time to work on it so hoping posting it here could give me the motivation to work on it.
tiLex v0.2.1 (http://bc.uberm00.net/misc/tiLex/tiLex_0.2.1.rar) (rar file 43kb, includes a test scene and texture)
1. Launch the script, select the object you want to paint on and press "Pick Object". The object should already have a UVMap and a material with a bitmap.
2. If there are multiple bitmaps in the material, a dialog will popup asking you to pick the texture. The object will then be converted to an Editable Mesh
3. When you click on paint, the painter interface gizmo will appear and you can paint on the object.
4. To choose the tile to paint, press "Pick Tile". A new dialog will appear. Drag-select the texture you want to paint.
5. If the texture is misaligned, the rotation/flip options can turn the texture the correct way.
6. When you're done, press "Return State"