Don't mess with firmware.
Instead install TTAV134's BIOS from here: https://archive.org/details/T43SLIC2.17 ... 0ByTTAV134
That certainly sounds safer, and that's probably what I'll end up doing in the end.
For future reference, though, I might have figured it. Does this sound right for me?
1. Download Lenovo's PATA HDD update tool (https://pcsupport.lenovo.com/dk/en/oldd ... s/ds002826
2. Find the FW.PRO file on the downloaded ISO and make a note of the line containing your drive, e.g. "HTS721010G9AT00","MCZIA41M","MCZIA5DA","MCZIA5DA.bdr","hfui15.exe","IA5DA","/u mpcA5DAb.tbl"
Details to note in my case:
MCZIA5DA.bdr: New firmware file
hfui15.exe: Flash program for specified harddisk
IA5DA: Version control phrase
mpcA5DAb.tbl: CHECKSUM file
3. Burn the bootable ISO to a CD
4. Boot the CD up on the ThinkPad in question
5. At prompt enter 'copy mczia5da.bdf mczoa5da.bdf'
6. Then run hfui15.exe oa5da mczia5da.bdr /f mpca5dab.tbl
7. Finally run hfui15.exe ia5da mczia5da.bdr /f mpca5dab.tbl