The DD-WRT v24 SP1 firmware supports the ASUS WL-500g Premium V2, but the V2 revision uses a newer hardware base (BCM5354) which has different USB hardware than the initial model. With USB support enabled, the v24 SP1 firmware will correctly detect and create device entries for USB storage devices, but will reboot when an attempt is made to mount them.

After cross-referencing the SVN change log and DD-WRT forums, it appears the fix went in sometime in August 2008. Upgrading to a NEWD build by Eko from September 2008 and on (I am using 10949, built 11/19/08) will add functioning USB support. I have only tested USB storage and not printers, webcams, etc.