During a good walk with my dog I figured out this simple solution. When double-clicking in the File Explorer on an .apr with spaces in either file name or path, the whole path + name is passed on to Arcview as a list, with the spaces as list tokens. The list can be rebuilt into a complete file string using the Self-argument in one of the scripts that is part of Default.apr: Appl.initialize. As a spin-off, it is quite easy to have Arcview open .odb's or .dbf's in this way as well, by checking in Appl.Initialize what file type the user wants to open, and acting accordingly.
Hubert
Hubert