I had to send out several thousand a week (all opt in) for a coupon site, I just figured out how many seconds there were in the given time period I needed to send them, and generated a random number of seconds between 0 and that number, added the random number to my "SendAt" date in my queue.
Not perfect but it worked. I do agree that ultimately it's a flawed concept, if you really HAVE to send thousands a day you'll need to contact a mail service.
http://www.streamsend.com/
BTW, you need to be very careful about not getting added to any of the blacklists
http://www.mxtoolbox.com/blacklists.aspx
