Ok so I managed to create a signed jailbroken IPSW of 4.3 (I didn’t attempt to flash it but this is worth a try). There are a few steps so I will edit this post with them for you to follow…
I am going to start from the very beginning therefore if you have already done some of this then that is fine.
Step #1: Download the 4.3 IPSW
Step #2: Download and run Seas0nPass
Step #3: Hold shift on your keyboard and click Create IPSW
Step #4: Selected the downloaded IPSW file from step 1
Step #5: Wait for the jailbreak to be finished. When it is you will be asked to put your Apple TV into DFU mode. Ignore this and close Seas0nPass as you are finished with it.
Step #6: Download and run iFaith
Step #7: Dump the SHSH blobs for the Device
Step #8: Click Build signed IPSW w/ Blobs
Step #9: Select the SHSH Blob (will have the .ifaith extension)
Step #10: Open the following path on your computer C:\Users<USERNAME>\Documents\Seas0nPass. You will have a folder called Downloads. Copy the IPSW from the Downloads folder to the Seas0nPass folder. You will then have two IPSWs. One will have SP in it.
Step #11: Click Browse for the iOS 4.3 (8F455) IPSW. This file will be in C:\Users<USERNAME>\Documents\Seas0nPass and make sure to select the IPSW WITHOUT the letters SP.
Step #12: Once the IPSW has been verified (it does an MD5 checksum to make sure you are selecting the right file but this is the only time it does throughout the process). You need to rename the files as such:
AppleTV2,1_4.3_8F455_Restore.ipsw –> AppleTV2,1_4.3_8F455_Restore.ipsw_
AppleTV2,1_4.3_8F455_SP_Restore.ipsw –> AppleTV2,1_4.3_8F455_Restore.ipsw
This will force iFaith to use the Jailbroken IPSW that Seas0nPass created to sign.
Step #13: Once the signing is complete then follow the on screen instructions to put the device into DFU mode and restore the AppleTV in iTunes. Or without putting the device into DFU you can try to simply restore the IPSW in iTunes to see if it accepts it.
Good Luck and let me know if this works!