That's one usefull way of doing it, a method i have used before is to use the trims at full swing to activate a trigger, this is easier if your radio has ratchet trims otherwise it can be a pain.
my 14th scale Tamiya truck used the trim method so i ended up wiring shortcut switched to the pots inside with resistors to mimic the functions, most transmitters are 8ch anyway, just not wired, if you look at the output with a scope you'll see the extra 3ch's there just not doing anything.
the PPM signal is available through the trainer socket on the back of the transmitter for a non invasive method of getting the data, or get some nRF2401 modules, they are easy to work with and alot of cheap rc systems use them anyway