Hi Jurgen,
I tried using a workflow but it didn't work. Also I tired to run a custom action same result.
Both my attempts failed because the Approval action is a seperate process that has to be run after the item gets created or modified.
I wish I could help you out more and I spent time trying to find a solution for you but I just couldn't find any.
PS
I'd recommend you to take a look to a sample Visual Studio project that we ship with our web parts installer (you can find it under the product installation folder\Sample Code). You can accomplish lots of things using that sample (you have the power to write your own code that gets executed by the ListForm web parts).