Officially "no" and I don't think I've seen a hack for this before. Once upon a time I created a "guest account" that random visitors could use by guessing the login and password based on clues provided on the login page, but I don't have the hack required anymore.

Do you have direct contact with these potential users? Are they using private computers or shared systems? If you can get with them in person and they're using private computers then walk them through logging in and tell them to not log out.

Alternatively, and again only if you have direct contact, you could suggest they use their first name as login ID and their last name as their password. It's not the safest credentials in the world, but I'm thinking it's not likely they'll forget those to pieces of info.

You could do what demo.b2evolution.net has done. Create a user account with limited permissions and then give out an url with the login info in the url. So, when people click on the link (or their bookmark) they're logged in and taken to the Write tab.

Your url will look something like this:
yoursite.com/admin/?log=demouser&pwd=demopass

Just change the green parts to match the account you create. If you privately email this link to people you should be ok, but if you put a link on your page, you'll probably get spammed.

why don't you just simplify the login stuff so instead of;

Login:
username: [ textbox ]
password: [ textbox ]
[ Login ] [ Register ]

You have:

Who are you?
firstname: [ textbox ]
lastname: [ textbox ]
[ Done ] [ You don't know me ]

And just have that in your skins sidebar instead of in a seperate login page....

Sounds simple and friendly to me....