You could let people pass the username/password in as part of the email tag, like:
guerillaemail+myusername-mypassword@gmail.com
As others have clarified, only for complete throwaway accounts obviously. I tested '-' and '!' and those characters appear acceptable as the delimiter. ':' gives a very strange error on send when using gmail.com. Also imagine you'd lose some letter casing along the way.
Really just a fun and terrible idea!