I'm trying to populate a paging grid with a list of all currently open tasks

Try the Portal to SAIL plugin available under Shared Components.
Hope that helps!

Hi I achieved almost same requirement... Following are steps you need to take.
a. Create One Custom Task Report in which display all required task.
b. In one process Model Use Execute Custom Report and configure above report.
c. Save Information in one CDT which is going to use in Paging grid and display in the Form.
d. Also save task id and Process ID of all the record in array PV.
e. In form use 2 button Reassign and Approve. whenever button clicked ..Check Paging grid one of the record is selected using JavaScript. Save the selected index in one array pv.
f. With those Index Value we need to get the selected proccesID.
g. Configure Send and Receive Message. Send Task Action Value 1 for Reassign and 2 for Exception.
h. At receiving End Check , Configure Condition on Escalation if sent value is 1 the Escalate task and reassign.
e. If sent Value is 2 then make exception condition true and configure approved flow through exception.