sorry no, it's not possible - The boot screen is much like the bios screen - there's no power saving features.
It's not "preventing" the monitor from turning off, it's just not turning off the monitor - it's a function of the OS itself, not the screen.
it's a pity to see all these screens turned on awaiting for someone to logon.
would be a nice feature to implement (screen goes black after x minutes)
anyway, thanks a lot for the prompt answer !