Indeed, I don't believe there is any perfect way to support filenames with spaces. Double-quoted "foo bar.tex" is the most likely. I don't know if that works in miktex. Multiple consecutive spaces is even harder, e.g., see thread starting at http://tug.org/pipermail/tex-live/2016-May/038331.html.
I believe luatex has enhanced the \input primitive so that if the next character is catcode 1, the filename is terminated by the next catcode 2 token. I'd like to do this in all the engines (except "tex"), but even if we did, and got christian to do it in miktex, it would still be a hassle in texi2dvi to detect whether that support is present. :( -k
