In FAQ #17, there's a passing reference to "an email address that has previously been validated on the account", and also "If [the email address you entered] is currently registered or previously validated...."
But, I'm thinking it would be nice if this were stated explicitly in FAQ #17, maybe even under its own subheading, thus:
"I no longer have access to the email I used when I set up my account.
"LiveJournal will send a password reset message to any validated email address associated with your LiveJournal account. This does not have to be the email address that was originally registered with your LiveJournal account, nor does it have to be the currently registered email address. To have a password reset message sent to another validated email address associated with your LiveJournal account, enter that email address in the Lost Information Form."
I've checked, and the "any validated email" doesn't seem to work for username reminders, only password resets. I don't know if the email address needs to have been validated for a minimum period for it to work for password resets.