Look in the EngVar, there are some IntPtr like hwnd, hwndparent, hwndtarget and pRectTarget (which points to some RECT struct afaik) - they should do the trick i think. I'm sorry but i didn't work with windows forms yet and thus can't provide you with any help reagrding this topic at the moment.

get the C# wrapper:
for A7.85.4 and A8.30.4, Version 2.3.9
at http://acknexwrapper2.matthias-auer.net/ or visit the thread