First off, I’d like to thank everyone involved in developing KiCad.
To my understanding, spaces in label names are possible if created in the schematics editor. However, it appears that the dropdown menu for filtering net names in the pad properties window doesn’t allow spaces in names, when choosing the create new option from the list. Is it possible to add that functionality?
Your (and mine) current designs will not be broken, because spaces will be left allowed in schematic files itself, but there will be no possibility to enter spaces by hand when using eeschema GUI.