Calling texlua from texlua, with a space in the tex binary path

I am investigating this in connection with my epspdf texlua script,
which calls luatex at some point. A user who had installed miktex
under Program Files ran into a problem. When I ran my test suite --
another texlua script -- I ran into more problems.

So I wrote a texlua test script which calls another texlua script in
the same directory and ran it with portable miktex, with TL/linux
and with TL/w32. Note that TL is portable out of the box in all
respects that matter here. None has its binaries on the searchpath.

The test script tests 4 cases:

1. texlua not on the searchpath; use os.execute with full texlua path

linux ok
tl/w32 ok
miktex:
  luatex --version: ok
  luatex with script: message 'x:\mik' is not
    recognized as an internal or external command,
    operable program or batch file

2. texlua not on the searchpath; use os.spawn with full texlua path

linux ok
tl/w32 no output, not even with luatex --version
miktex no output, not even with luatex --version

3. texlua on the searchpath; use os.execute

linux ok
tl/w32 ok
miktex ok

4. texlua on the searchpath; use os.spawn

linux ok
tl/w32 ok
miktex ok

Are 1. and 2. even supposed to work on Windows?

Testfiles, plus the above description, available on
https://www.dropbox.com/s/4ezask7isyor195/tlu_call_problem.zip?dl=0

-- 
Siep Kroonenberg