hi,
so, do you use T89C51RD2 (Temics) exactly or AT89C51RD2 chip?
If you use T89 then its factory bootloader is placed inside program flash memory (in upper 1kb of it). Could you just write a program which "downloads" it with MOVC commands? Then search for echo feature and NOPs it with another piece of own code which "uploads" some bytes into bootloader area. Here please note, that you need to set BLLB of Hardware Security Byte to have access permission for frogramming that area.
BTW, I do not see why do you need such thing? Echo is a way to control that all bytes are received correctly - do you need to disable it really?

Regards,
Oleg