I may well be wrong but thinking about it further, if you were to gain a copy of a 'stand alone' Leopard install dvd, you could try starting up from it and attempt an erase and install.
you'd soon know from the initial screens whether or not it were going to be possible.
if it will allow you to proceed - and of course you would have backed everything up to an external hard disc - you would end up with a basic installation of Leopard and would then need to hunt down iLife, iWork, etc, from elsewhere.