Having just received a cheap but very able RC system, the Turnigy 9x (and 4 extra EX), I set about modifying it even before I have used it in a model:
Done so far
1 - 1st mod was the addition of a tiny 8.2K resistor to allow the use of Lithium battery as explained by Bruce. However in my case I used AA size 14500 lithium, just two of those in the original holder plus some dummy battery to make the circuit, I plan to eventually use 2 AA battery holder instead.
2 - Solder an additional 2 pin power socket (RED) as used in RX battery circuit etc. this will make it easier to add the 2 AA holder as above, this type of connector is sold in RC shop for making up small battery pack, easir to find than the original 3 pins.
3 - Remove the original charging socket as these can't be used for the Lithium, instead I created a 6 pin round port for the connection with an external AVR programmer, this is a passive connection only. The round PC keyboard/mouse style socket fits really well into the charging hole.
4 - Make the antenna retractable ! Yes even with the short 2.4GHz antenna I don't like the way it stick out on top which make it more fragile when transporting. So I added a length of tube inside the case and push the original antenna back, when in use just tip the TX down and it will drop out a bit, then pull it to make it come out all the way, the springing plastic fits just right inside the antenna receptable in the TX to keep it there by friction. You have to remove the original mounting hardware of the antenna, this is easy as they only clip and screw in, no cutting.
5 - Shorten the stick, to fit my thumb better (cut with a Dremmel)
6 - Make the 4th stick channel also center springed (by shaping a piece of plastic similar to those used in the other 3 fucntions)
7 - Fitted the backlighting panl bought at the same time from HobbyKing (this is LED and does not need any noisy driver, simply run off 5V)
To do
8 - Solder on the connecting wires for the programming port
9 - Reflash it with custom firmware as and when I bought the programmer, will then remove the resistor in 1 as the alarm voltage can be adjusted in the custom firmware.
10 - Replace all the internal voltage regulator with low drop out type, however I will do this only after making sure that the TX has worked well for a while, just so that I know any problems are not due to this.
I think the above conclude my requirement for a custom TX and thankfully the Turnigy lend itself into it very very well, they way its made make it easy for conversion (heck even the main board has pads to solder on the programming interface) and the fact that it is cheap so I mod it it without too much fear.