Can Keychain save login details for the popup style login boxes?

I'm in the process of saving all my login details with Keychain. However I'm not able to save passwords for the popup style login boxes (e.g. the login box you get when trying to log in to a routers' config page).

I can log in directly by creating a bookmark containing the login details (e.g. username:password@www...com) but then my password is in plain text. Is there any way I can get Keychain to autofill this box or can I alter the web address somehow so I'm shown a form instead of a dialogue box?

Thanks,

Jake.