I love that Sitemap Generator has the ability to log in via username/password forms. Is it also possible to log into an IP address?

There should be no difference on IP address and normal website addresses. I don't actually remember explicitly using login with IP addresses, but it should not be a problem. If you experience any problems, please email support with an example!

Here is the general help for login capabilites in sitemap generator:
http://www.microsystools.com/products/sitemap-generator/help/website-scan-login/
(As documented in the help, this will work for many websites, but there are some where website login is very hard to get working.)