Thank you for your post. I am not a developer and don't really understand php. Having said that, I think I figured out where to tweak the code. I believe this tweak will indicate which online users shown on the homepage are teachers or administrators. I think admins are hidden when in a course, but teachers will show up in their course with the added icon.
Others might step in and correct me. I made the following changes in my localhost 1.5.3 and 1.6.1 Moodles. You will need to have access (probably as a site administrator) to the file \moodle\blocks\online_users\block_online_users.php .
Between lines 90 and 100 you should see a line something like:
$teacher->fullname = '<strong>'.fullname($teacher).'</strong>';
change it to
$teacher->fullname = '<strong>'.fullname($teacher).'</strong>'.'<img src="'.$CFG->pixpath.'/i/info.gif" style="vertical-align: middle;" alt="';
This inserts the blue info icon after the bolded name for an instructor or admin person. I would think adding an file of the same size to the \moodle\pix\i folder with a different image, and changing name in the code to that image might work.
Hopes this helps. I attached an example.