look, you described well what you want.
You know how to catch a mouse event in Qt, you know how to set a mask.
Now use google and MSDN to figure out how you can send events using WinAPI.
You are not the first person to want to synthesize mouse event win WinAPI, so google should be pretty helpful.
CodeGuru is a good place to ask C++ and windows programming questions.
You can't expect us to feed you with everything, or do the searching for you, specially if its off topic. (not Qt)
If you have specific questions post them.