I'm really interested in a mod (or would that be two mods) that would make it so that avatars and signature lines could be made hidden to guests. Is something like that possible? :)
This can be done via SMF. However it would also set them off for default for new members.
Admin > Current Theme > Reset Options > Configure options for guests using this theme
Check to disable avatar and signature.
However if you want an alternative method that will only do it for guests.
You can do this, however it will only affect the Topic/Posts display. It won't affect profiles etc.
Open Sources/Display.php
FIND
$memberContext[$message['ID_MEMBER']]['ip'] = $message['posterIP'];
ADD AFTER
if(!empty($context['user']['is_guest']))
{
$memberContext[$message['ID_MEMBER']]['avatar'] = array();
$memberContext[$message['ID_MEMBER']]['signature'] = '';
}