I misread you before.
AFAIK what you want is not possible, you can only have the countdown bar on the screen that shows the "default partition".
You can use Instant Menu=y to go straight to the graphical device selector. But Instant Menu=y will override the timeout setting..so you'll have to manually select which partition to boot.
Looks like you can have your cake, but someone else will be eating it for now.
Oh damn, I AM going crazy, we're in the feature requests