In case someone starts to implement it:

Which one is the "Main" project?
Is it some new global setting or is it the Default Project project set in My Account > Preferences

In our hackery case it's just the first one:

$in_main_project = ($t_project_ids[0] == $current_projects);
echo '<form method="post" name="form_set_project" action="' . helper_mantis_url( 'bug_change_status_page.php' ) . '"' . ($in_main_project ? '' : ' style="font-weight: bold; color: red"' ) . ' class="form-inline">';