It's better to use one of the site URL template tags to point to the right directory.
Do you mean something like this?
<?php echo site_url(); ?>
If so I'm not sure I see how that would work—wouldn't that in itself necessitate at least require('wp-includes/link-template.php');? In which case it seems like a catch-22.
If you plan on putting your plugin in the repository, you shouldn't require wp-load.php or anything.
Do you have a citation for this? I couldn't find anything in the guidelines.
Besides changing the 'authenticate' filter to your own process, why not send your page's login data to wp-login.php instead of your custom page that needs to require wp-load.php?
Thanks for the suggestion, I will try this approach.