Sorry I disappeared for a bit.
Is this what you were trying to do http://mrjcd.com/EA8500_DD-WRT/ only with the stock firmware instead of DD-WRT?
I know you said you had a JTAG, is it also like Gallo1961 mentioned a TTL as well? I don't know what the voltage is on the TX/RX on the serial connection on the EA8500, on the WRT series it's 3.3v.
Just trying to maybe give you ideas as to why it isnt working for you.