AT89C51RD2 manual says:

The 64-K byte Flash is programmed by bytes or by pages of 128 bytes. It is not necessary to erase a byte or a page before programming. The programming of a byte or a page includes a self erase before programming.
...so it is possible to program any byte without erase full page.

1) Benny/Thomas, whatever his name is states he uses a Philips chip.
2) I have no idea what the AT89C51RD2 does, but the AT29C020 allow you to write without erasure, it does, however, when you are done writing erase a full page. I believe thay are doing some trick with an internal RAM.
3) Philips are spreading some rumors about byte erasable flash but I have yet to see a chip but, for now, the rx2 is 4k pages.

Erik