In Identity Manager, we are able to make changes to attributes included in the approval work item.
Is it possible to have the same functionality in Identity Portal, allowing the approver to make changes to the proposed attributes?
To allow the approver to make changes on the approval form, make sure you expose the approval task you want to be using for approving these requests via TEWS, then attach a form in Portal to this approval task.
You configure the form in the usual way, whatever fields you want to be read-only and whatever fields you want to be updatable by the approver.
When the approver accesses the request for approval, she can go to the Process tab to modify the form data before approving.
In addition, you would want to make sure your IM approval task profile contains the attributes you want to be modifying during approval.