Tbh I doubt this is possible, you are not dealing with a standard FC with pwm outputs to control a gimbal, DJI have very intergraded system and internal comms are extremely had to find, while I won't say it's impossible it's going to be very close to being.
The simplest and easiest and least risky thing to do would be to get an Taranis and a separate receiver and make a small module to do what you want with integrated power that just attached to the craft, something like the XSR receiver is small and got decent range, what you looking to control ?