Nice workaround, thanks! But yeah, looks like you lose it if you save any of the settings, not just the Signup Form Settings.
Here’s what I tried, and so far it seems to be working. In PDb_Settings.class.php, in the plugin_settings block for ‘Duplicate Record Check Field’ (around line 311), I added _get_identifier_columns() to the options array:
'options' => array_merge($this->_get_display_columns(), $this->_get_identifier_columns(), array('Record ID' => 'id')),
That adds my hidden user_login field to the menu.
I’ll report back if I see some problems with this down the line. OR do say if you think this could be a problem.
thanks!
–steve.