The 904HD are supported.
You have to install it with the Classic Installer, follow the Debian Guide, in depth see Chapter 4, Section 3 - Preparing Files for USB Memory Stick Booting. The wireless works only with Madwifi, you can see DebianEeePC/HowTo/UseUpstreamAtherosModules for download the latest driver available for the Atheros wifi chipset and see DebianEeePC/HowTo/Wifi to configure it.
I don't understand why the custom installer would not work for this model. It ought to. Would you please explain? -- BenArmstrong