As a side note, I was worried about the integrity of the data I'd read from my 27C512 directly into the Pi as the Pi's GPIO is all 3V3 (and not 5V tolerant) and the CMOS 27C family is a 5V part. In my setup I'd powered the 27C512 from the Pi's 3V3 bus. To satisfy my curiosity I purchased a TL866II USB reader from eBay.co.uk which has just arrived. I've just imaged the chip again using the 866 and compared with my original Pi-derived version and can confirm they're identical. So for the 27C family at least it looks like there's no need for level shifting logic if all you want to do is read using a Pi...
Code:
[cxf@ux303 EPROM Reader]$ md5sum elektor_scms_rom.bin
64bd11d7cd3a2ffcde307e94fd8fd679 elektor_scms_rom.bin
[cxf@ux303 EPROM Reader]$ md5sum TMS27C512@DIP28.BIN
64bd11d7cd3a2ffcde307e94fd8fd679 TMS27C512@DIP28.BIN