Hey Andy - I don't believe this extension is built for this purpose. Sending mass emails is a lot trickier and there are several other systems available for this purpose. (I won't suggest any here at WA Forums). Simply looping through the database is sure to time out, then you won't have any idea where to pick up the mailing from. With default PHP.INI settings, my experience says you might get 500 emails off if your lucky before this timeout occurs.
I believe the intended purpose of UE is to send a few emails, not mass emails. Now I could be wrong - but I don't think so.
The biggest problem you will face if you try to send Mass Emails like this, is your IP address is sure to get blacklisted at many large ISP's and FreeMail services like "Hotmail, AOL, Yahoo, etc..."... There is a lot to consider before offereing these services to clients.
Just speaking from experience :)