If you setup screen sharing on the MacBook you MIGHT (underscore might) be able to access System Preferences on the iMac. If you can then you can turn on screen sharing on the iMac and be able see what you're doing on the iMac. However I'd really recommend taking the machine into an AASP to see if your black screen on the iMac is a software issue or hardware.
I'd also like to know. This is the same thing i'm doing. I'd rather hook a $300 monitor to it then pay $700 to have the one that broke be put back in it. Atleast if the external breaks i can replace it for 100 bucks.
Is there a button you can push to send your destop to the external monitor? I've seen in done on laptops when people are giving powerpoints on projectors. Please tell me Apple has exceeded this and all i have to do is think about it and it happens.