Dear Philippe,
Thank you for your query.
I am not aware of any such possibility, and I am pretty sure that the Revit API does not provide any support for this either.
However, I am also pretty sure that you can easily achieve what you are after using the Windows or .NET API.
I would suggest searching the Internet for solution to suppress a flash screen.
You might be able to use some of the techniques described in my stand-alone app to detect and close an arbitrary Windows form:
http://thebuildingcoder.typepad.com/blog/2009/10/dismiss-dialogue-using-windows-api.html
I hope this helps.
Please let us know how you end up solving this, in case anyone else runs into the same requirement.
Thank you!
Best regards,
Jeremy