Is there any clean way to have the Drupal login form create a session for phpBB also?
If what you want is a clean way to integrate phpBB with Drupal, I think phpBB Forum Integration is want you need.

This module will not only "embed" phpBB within a Drupal site (and synchronize account info, such as user names, roles, passwords, emails, signatures, and avatars, with only one login), but it also let phpBB topics be created from Drupal nodes.

The phpBB forum can be displayed in a full window, in a frame inside Drupal, or in a Drupal page.