Thanks, Jason!
I've had a look at the "Purchase order" demo app.
But the difference to my application is the fact, that in the PO application multiple POs can be approved.
In my application users will only see a single request that they can approve or decline.
So there's not really listed data to be shown (like the PO list).
Maybe it's better to design it as done in this blog: BPM OData: Implementing an advanced custom task execution UI.
What do you think?