I read here:
Installing on an ASUS WL-520gU
Installing from OEM firmware
Installing Tomato firmware from OEM firmware may need a little massaging to get it on the device.
1. If the device has OEM firmware version 3.x or later, then you need to revert it to a pre-3.x version, first (download from support.asus.com.tw).
2. Downloading and unpack the "ND" version of Tomato firmware, rename the file to "WL520gu_22.214.171.124_EN.trx".
3. Use the OEM menus to update the firmware with the renamed Tomato file.
Once installed, you can load any other firmware file without these machinations, using the config menu item to load new (or old) versions of firmware, including OEM versions.
My WL-520GU is running ASUS firmware Version 126.96.36.199_RU (ASUS WL-520GU firmware in English/Russian). This is the latest ASUS WL-520GU firmware I can find that supports English.
Q1. Do I really need to downgrade the ASUS firmware below 3.x as stated in the Wikibooks post above to install Tomato-USB? I see no reference to this requirement on the Tomato site (sparse documentation) or here on the Tomato-USB site (searched site, Wiki, and Forum).
Q2. I have put the likes of DD-WRT on this WL-520GU in the past. I seem to remember doing this with Pumpkin TFTP server/client (http://kin.klever.net/pumpkin) from WinXP-Pro. And I can easily revert to ASUS firmware via the (wonderful) restore button on the WL-520GU also via TFTP. Will this also work with Tomato-USB.
Q3. I have seen scattered on the Internet that Installing the likes of Tomato-USB may require clearning NVRAM content (via The Tomato-USB WebGUI?) If I do this will I also clobber the (what I assume is) the native ASUS WL-520GU's bootloader that works with the Restore switch on the back panel.
I plan to first try a K24 version of Tomato-USB which this page:
Implies all K24 Tomato-USB versions will work with the WL-520GU (this is build 54 at post-time).
It's been a long time since I put third-party firmware into this WL-520GU, but I'm having problems with NTP (IP/UDP port 123) port forwarding with the ASUS firmware. And it seems Tomato-USB is the best option to try as an alternative.
Sorry for the long post. Any replies are much appreciated.