Hello there,
Unfortunately, currently there's no way to apply custom default order other than modifying base epesi file. This means you'd have to apply this solution every time you update epesi.
If you want to achieve this, edit file modules/CRM/Tasks/Tasks_0.php and edit line 25:
$this->rb->set_default_order(array('deadline'=>'ASC', 'longterm'=>'ASC', 'priority'=>'DESC', 'title'=>'ASC'));
First value in this array is most important so you'll have to add your field there.
We have a plan to make the browse display fully customizable on user level (and defaults on admin level), but we can not give any estimate on when this feature will be included.
Hope this helps,
Arek