Assuming that the physical interfaces are sorted you may be able to transfer bits from one pin to another which I think is what you were suggesting. Some nice tight code should be able to do this.
However, unless you reassemble the bytes the only way to detect the end of the PC's transmission is with a timer. If there has not been a falling edge on the signal for say 9 bit times then it's done and you turn off the Tx pin to the 485 and hope the controller isn't to fast to respond.
Rob