Oh, forget about it. As I should have expected, the error is on my side. I used cpuspeed from the firmware 1000 tftools archive and yes, I'm still on that version.
I was having issues using my Xilinx Platform Cable (the "Programming Properties" dialog on iMPACT insisted in being blank, with no options), so I followed
this article, about generating a SVF file before programming the flash. Apparently, it just pretends to be writing...
Could it be that the CPLD is write-protected?
Back to square one, I guess... Sorry for taking your time.