Symptoms
How to get Workflow 'Client Event' / 'User Task' Destination Users via SmartObject List method?
Diagnoses
Possible with a Join() Inline Function:
http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_Text.html
and
'Plan Just Once' with a single slot:
http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_PJO_Scenarios_and_Outcomes.html
Resolution
Create a data field with a Join() Inline Function. Use Join() to take in the multiple outcomes returned by the List method and concatenate them with a semi-colon character "" as its delimiter, returning a single string containing all the users.
The Destination Rule will send out tasks to all the users, but when someone actions one they will disappear from all Worklists.