How to set the equivalent of Region on a ToolStripItem

Answer 1

You need to create you own ToolStripRenderer, and paint buttons in your code. With such approach you can simulate any shape, and in general almost anything.

Then after custom renderer is complete, you will need to assign your renderer to toolstrip and thats it.

An article describing writing custom renderers.

Update: You can also check this article, it can be useful for your task.