Wow, I've wanted something nearly exactly like this for a long time. But for me "Send Later" is one of the lowest value functions (I think gmail already has a lab that does this). Instead I like an email re-appearing in my inbox if no-one has responded to it in a few days so that I can then follow up on it. Their email game seems pretty nice too, although I wish it had keyboard shortcuts as well.
With less than an hour of work, I bet you can modify the script to make exactly what you want. Actually, it seems that most of Boomerang's functionalities could be substituted for free with a couple of Google Apps Scripts.
