hi mate,
here is a cheap but not versatile solution which I use on 8051 systems:
With a universal programmer (GALEP III) I program the code to a
FLASH-EPROM (AM29F010). Then I use an adaptor socket that makes it
look like a 27C512 to the µC.
I think you can also get 27F512 FLASH-EPROM.
But as I said: not very versatile. You'll have to use different adaptors
for different EPROM sizes.
If emulating a 256Kbit EPROM ( 8 bit only)will suffice, the one in Elekt0r january 2001 is pretty fine. It works off the serial port and is based on a at89c2051. Source code and PCB is available at their web site. and the mag itself is available on another thread around.