According to some people on some devices the D+M menu is bothe on flash and rom:
D+M+reset = load diag menu from flash
Fn+D+M+reset = load diag menu from rom
Can you try the Fn+D+M+reset before restoring the original boot loader stuff?
Bufore stating that it's not working make sure you also try this:
remove battery and power adapter
pres and hold Fn+D+M
while still pressing the buttons insert the power adapter ...
There is a restore procedure for the diag menu:
1) put a copy of the originan NAND BACKUP on a CF or CD (something like SYST<model>.DBK). If you do not have one you can get it from http://trisoft.de/en_downloads.htm
2)boot the emergency system (press OK while power on)
3) insert the device where you have the nand backup and cd to where ever that is
4) nandlogical /dev/mtd1 WRITE 0x00000000 0x000DBFFF <your nand backup>
replacing <your nand backup> with the name of the original sharp nand backup.
Your done with just the boot loader and other flash menus ... not not the OS yet.
You're not using a c760 I bet ? ... on the 760 the D+M menu does not get overwritten.