Program defaults

No. That would be nice though.

Yes.. takes some effort:

In your content type, where you added field you are multi-selecting against

select default value
select phpcode

and then enter and array that is appropriate:

I only need one value, and generate it via my own function return_current_users_showhost_id()(which returns a simple numeric value) like so:

return array(0 => array('nid' => return_current_users_showhost_id()));