The 89C51 can be programmed in-circuit - as described in the application notes I mentioned earlier.

However, it is far more complicated than modern ISP schemes that require just a serial port; eg, the 89LPC932.

Programming an 89C51 requires 8 data lines, 12 address lines, and 8 other control lines, and 12V for some (most?) variants.
One the Atmel Application Notes uses another 8051 to do the programming! So you could build that, and call it a "Programmer" if you want...