Your suggestion is a new one to me, personally. I've searched the WEB faaaaar and wiiide and can't find 1 reference for using fabric softener for wallpaper glue removal. Oh, I did find one instance where it was recommended for heat-gun glue removal, but that was all. And I did find lots of references for fabric softener 'sheets' that you can use to remove things like cat hair from clothes and static/dust from the TV screen.
If you have a place for me to read, I'll check it out and certainly keep it in mind for a recommendation! The keywords I used in my search were fabric softener and glue and remove. (Maybe I need to use some other combination, I don't know.) Also, what's the mix? And do you need to clean off the walls after you use this mixture?