Custom firmwares such as Tomato or DD-WRT do not support modem/router combo devices. They don't have anything built in to control the modem portion of the device, so even if you got it to flash without bricking the device (The device in question does have a broadcom chip that would normally be supported), you would no longer be able to use the device as a modem.
If you need the features that Tomato or DD-WRT provide, you may be able to disable the router capabilities and firewall of the Aztec (just use it as a modem) and purchase a router that supports one of these firmwares.