Using the IHtmlDocument2.designMode property set to On to switch a WebBrowser control hosted on a Windows Forms form to editing mode suddenly stopp
Yesterday, Internet Explorer 9 RTM finally was released.
I did some more tiny adjustments to my control, but basically the idea with the intergrated, small web server seems to work rather well.
So the solution is in this Code Project article:
Zeta HTML Edit Control
A small wrapper class around the Windows Forms 2.0 WebBrowser control
This was the only solution that worked for me.
I hope it is OK to answer my own question and mark my answer as "answered", too?!?