A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Sorry, I don't have a solution for you despite my best fiddling but I'm throwing your reply a +1 for thinking outside the box. I'm a big proponent of using the available resources to the fullest extent and your number manipulation within a custom number format was impressive.
I found that the comma does not appear to be lost during the save, reopen process. Rather it is lost immediately when Excel stores the custom number format. You are able to achieve and retain the displayed 'conversion' because that seems to be determined before the number format is stored. The only option I see to retain it through multiple sessions would be to rewrite it in a Workbook_Open macro.
Again, well done despite the limitation(s).