This can usually be achieved on the EV3 with a bootable SD card - you would set that up with a new firmware image and a loader that copies it across to the brick.
The leJOS team have ported their codebase to the EV3 - so with a bootable SD card you can indeed load a modified version of the firmware onto your EV3 brick.
You can then use their WiFi stack to communicate with your brick via a USB dongle - whether they support 802.15.4 yet (or ever will) I couldn't say.
The WiFi stack is loaded to allow connection to the EV3 via a supported USB WiFi adaptor. This allows you to connect to the EV3 via Telnet, SSH and NFS and makes program development very simple.