The current Jetson using Tegra K1 is a good example of what would be needed beyond support for the Tegra chip itself. The Tegra3 and K1 share similar needs and architecture. In order for L4T to support the Jetson specifically, firmware and boot loader specific customizations for the surrounding board (especially memory) has to be added. Memory itself and clocks have timings which have to be customized before the hardware is even capable of operation without error.
The command send fail itself means that this version of L4T doesn’t understand the flash process in your Tf700. Going forward in time, something like Jetson has software built into it which activates when powered up as the reset button is held down. I don’t know if your Tf700 does this, but then the next step also has to work: A piece of software inside of the board must provide something equivalent to a mini boot loader itself (in Jetson the 3pserver seems to do this job in combination with a fastboot image). Lacking this, you’d need something like a JTAG debugger capable of programming the device.
Embedded devices are not like general desktop PCs so far as loading operating systems goes. Your desktop has either a BIOS or its equivalent which presents a uniform interface to install software, and which reports about hardware and does initial setup. Embedded systems do not have this, and as such each system requires board specific support and programming. Imagine if every motherboard for every desktop required a custom installer and you’ve just imagined the embedded world and the Tf700.