Problema resolvido: a resposta é ... Ao contrário do GRUB legado, o GRUB2 começa a contar partições de 1, não de 0 .
A chave eram as seguintes instruções de como "converter" entradas menu.lst para 40 entradas personalizadas, tiradas do seguinte link
The following entries from the GRUB menu listing must be changed for them to work:
* title is changed to menuentry. The line must end with {
* root is changed to set root=
* kernel must be changed to linux
* Any partition designation (sda4, sdb6) must be changed, as GRUB and GRUB 2 count the partitions differently. The first partition for GRUB 2 is 1, not 0. Devices still start the count at 0.