When it worked, for me, the progress bar when to 100% and it was OpenWRT on the next reboot. If you are not rebooting into OpenWRT, you will probably need to follow the UART method described above.
The stock firmware can be updated with other versions of the official firmware releases by Linksys over TFTP over ethernet during the stock firmware boot sequence. Possibly try downgrading to the earliest version of stock firmware and then try flashing? The memory layout of this device seems quite complicated.