Hm… interesting. Not the answer but I found this while searching google.
http://www.dd-wrt.com/phpBB2/viewtopic.php?t=24046&view=next&sid=63992ba4c24fc0eb5da53f673cc8e020
"nvram set ath0_wpa_psk="your_key_without_quotes"" for WPA personal or "nvram set ath0_key1="as_above"" for WEP - not sure about the other encryption methods though. Check "nvram show | grep ath" for more options!