VS 2008 Working Within A Host Program

I am working on an addin/plugin for CAD software called Solidworks. I use it to batch process files (print, save, etc). One thing I'd like to add is a form that contains a stop button to stop the loop. The problem is that even if I use a background worker like I normally would for a standalone application, it still locks up when Solidworks is processing files. That's just normal operation for the program, the UI freezes when doing a cpu intensive operation such as loading or saving files, regardless of how I write my plugin. So my limited attempts at threading to get a form that won't freeze have been unsuccessful so far.