It doesn't matter if the /boot partition is the first or last partition or somewhere in between.
When your /boot partition is sda5 then change
set root=(hd0,1)
- to -
set root=(hd0,5)
#!/bin/sh
exec tail -n +3 $0
menuentry "Plop Boot Manager" {
set root=(hd0,5)
linux16 /plpbt.bin
}
See the Grub2 documentation for details.
Best regards
Elmar