Okay, so having tried the programmatic route and got stuck, and investigated it, I realised that I’d been dumb.
There is an UpdateTask workflow activity
Okay, so I missed the obvious – that to update a task would require an activity if it is to work in SharePoint Designer. However, I tried adding this to a workflow. When I went to the workflow code, I got the error:
Code generation for property ‘PercentComplete’ failed. Error was: ‘Property accessor ‘PercentComplete’ on object ‘Microsoft.SharePoint.Workflow.SPWorkflowTaskProperties’ threw the following exeception: ‘Object reference not set to an instance of an object’
Phew. Nice error. Then, sometimes, I managed to get the Designer view to have errors – it couldn’t draw the workflow. And build failed – the workflow designer.cs file lacked the line defining the private variable referencing the UpdateTask object. Oh, and trying to set properties of the task to update – more errors.
The short is, in Beta 2, Update Task doesn’t seem to work. I don’t think that there is a lot I can do to fix this…