To manually set a URL for a specific network name in gconf (that's where wifi assistant settings are saved), this is what you'd need to do:

0. Ctrl+Shift+X opens a terminal
1. Find the name used to store settings for the network you're after (I'm using limbergwendt here as an example)

~ $ gconftool-2 --all-dirs /apps/maemo/wifi_assistant
/apps/maemo/wifi_assistant/Telia__Internet
/apps/maemo/wifi_assistant/CSWLAN
/apps/maemo/wifi_assistant/skerrywendt
/apps/maemo/wifi_assistant/limbergwendt2
/apps/maemo/wifi_assistant/limbergwendt
/apps/maemo/wifi_assistant/NOMAD
/apps/maemo/wifi_assistant/default_settings

2. Now set the URL to open (http://wendt.se/) for the network in question (limbergwendt):

~ $ gconftool-2 --set --type=string /apps/maemo/wifi_assistant/limbergwendt/options/url 'http://wendt.se/'

Done.