Setting session variables using one-time login link

Hi,

I am trying to set a few session variables on login, in order to log users in to a separate system. I have managed to do this for users who log in manually, by adding a couple of lines to user_login_submit in the user.module.
This has worked fine in all cases but 1, namely when the user logs in using the one-time login link, which has this form: domain.com/drupal/user/validate/userid/blablabla.
I found the function user_pass_reset, which seemed to deal with this case, but no code I add there is being executed. I want to set session vars for the user id, user name, e-mail address and login time (I did this by setting $time=time(); before the last login time is updated for the user). Does anyone know what I can do to make sure these vars are set on login via the login link? Which function(s) should I edit?
Someone recommended creating a module for this purpose, but it seems like a lot of work for a (seemingly) simple task.

Thanks