The datasheet hints, that there is a preprogrammed bootloader in the SST89 part. If so, it is certanly different from the NXP one; I remember reading through the SST appnotes back then and they had a dissimilar bootloader and their own PC-side utility for it. I already dug through my archive to find the relevant SST appnotes/software, but it appears that I have lost them in some of the "oh let's just leave this less important stuff behind" moments. Maybe archive.org could help.

However, as I have preached for years, I am quite certain that it's physically the same chip as the 'V51Rx2, so the NXP bootloader should work there too. You'd need some means to get it into the chip, e.g. either getting the SST bootloader (if still preloaded by Microchip) to work, or acquiring a parallel programmer (I am shamelessly pushing Elnec's offerings, both as a patriot and as a satisfied user).

I wouldn't hold my breath for too long, though. Mind that Microchip already tagged the SST89 parts as "not recommended for new design" and they apparently provide nil support (no wonder, just stating the facts).

If time permits, I will try to get some SST chips and play with them a bit; but in the last few weeks I used the "if time permits" way too many times... :-(

JW