Martijn,
No, I haven't done this but I would expect this to work. Which angle are you working from? The supply of finished goods or the demand for raw materials? This would be a very easy thing to test so I kinda thought your question was going somewhere... I can hazard a few guesses from memory:
The only problem you could face is with the quantity. ECO's sit in the F4801 but the Order Quantity may be zero which mean that they would be included in P4021 in theory, but with zero quantity (meaning that it probably wouldn't appear in practice).
As for the demand of raw materials, I believe the ECO doesn't use the F3111 table so maybe P4021 would ignore whatever table it does use. Therefore I suspect your wouldn't see raw material demand.
What do you want to see? It is generally accepted that you do not want ECO's in the Supply/Demand Inquiry!