Old thread, I know.

Be on the lookout for the same thing. Found this free ware which facilitates my work. It creates thumbnails of the g-code files, but you do not have to start some special application. The thumbnails appear in the file picker on all applications and the Windows file explorer.
Not perfectly adapted to Mach but takes the standard code, and that's enough for me to recognize the program.

NcThumbnail dowload
http://www.microtechstelladata.com/Details.aspx?id=63