If you're locked out, you're locked out. Yahoo likely won't care unless you're a paying customer. Basically, if you don't have access to the alternate email on file then you really have no chance.
This is the details from their page:
You can get a new password, a Yahoo! ID reminder, or both. If you forgot
your Yahoo! ID or password, go to the
Sign-in Problems page.
If you forgot your Yahoo! ID, you need
to provide three pieces of information
so we can confirm your identity:
Your birthday
Your ZIP or postal code (at the time you created your Yahoo! account
or when you last updated it)
Your alternate email address -Note: You must have access to this email address
Country code (at the time you created your Yahoo! account or when
you last updated it)
Be sure to provide the same
information you gave the last time you
registered for a new Yahoo! ID or
password or the last time you updated
your account.
If you forgot your password, you can
create a new one. But first you’ll
have to provide the following
information so we can confirm your
identity:
Your Yahoo! ID
Alternate email address, mobile phone number, or the answers to your
secret questions
A Reset Your Password link will be
sent to your alternate email address.
Click the link and enter your new
password in the Reset your password
page.
If you entered your secret question
incorrectly or forgot what you
originally entered, for security
reasons, we cannot tell you what your
Yahoo! ID is or let you create a new
password.