Phil,
Unfortunately as I say on my website I am not a software expert so I cannot say regarding if the software failing would cause it. I would say from a non-expert point of view it is highly unlikely, because although it may not add up to you, from a hardware technical perspective (mine) I have not seen a faulty temperature sensor on any TiVo motherboard to date. Furthermore a faulty or misreported temperature sensor due to either the temperature sensor failing at hardware level or a software glitch will not stop a TiVo booting past the Boot PROM screen. A faulty temperature sensor will still allow the hard disk to boot as far as the "Starting Up....." OzTiVo screen (ie the screen AFTER the Boot PROM screen) but will stop the TiVo with the overheating warning message. The fact that yours won't even get past the Boot PROM screen indicates something more serious at a hardware level.
As Zollymonsta said I would be checking that the fan is actually working as a starting point. Further to this you can monitor the temperature yourself either from the "info" page via TiVoWEB or simply in the System Information screen from the TiVo on-screen menus. What temperature does it report?
The point I am making here is that it is pointing to a defective hard drive and the major killers of hard disks is overheating. Given your TiVo has reported an excessive temperature it is all pointing to a temperature related issue to me why youhave had what looks like a premature hard disk failure.
Until you can say the following two things there is not much else I can offer:
1. Does your TiVo temperature report getting anywhere near 60C
2 Does the 6GB hard disk still give bootup problems (and/or temperature warnings) or does it fix your issues?