I want to be able to customize the DHCP server, like customizing at which gateway IP a client mac is configured
Tale a look at this, is it what you mean?
Download site http://www.4shared.com/dir/v1BuINP3/Toastman_Builds.html (inc. torrent downloads)
Make a Paypal Donation http://toastmanfirmware.yolasite.com/donations.php
The article was written for Tomato, so yes.
My Tomato utilities site: http://multics.minidns.net/blog/articles/tomato_utilities