in the early days of PS3 jailbreaking I had stealthed the first backup manager for a German board where I was a moderator for the PS3 board.
It is not enough only to sign the backup manager with a demo PARAM.SFO. You also have to delete all files in the following folder on your hdd0:
The files in this folder contain the history of applications which you have started.
It could be that multiman automatically deletes the files when you exit multiMAN but I'm not 100% sure.
This is just a little hint for you.