I FOUND TWO SOLUTIONS!
TextSoap - new version #7 also allows you to striketrough text system wide.
Keyboard Maestro macro maker which a lot of people are using in replace Quickeys works too.
There is an example macro in the Macro Library, in the Text section, called Red Strikethrough that copies the text, applies red foreground, and strikethrough attribute to the clipboard, and then pastes the text.
Insert that macro from the macro library and remove the red foreground, or create it yourself:
Type the ⌘C Keystroke
Apply Style to Clipboard: Set strikethrough to solid single.
Type the ⌘V Keystroke
Hope that helps.