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Well, I don't know much about how the binary installer
works, but...

1st: shouldn't it be possible to involke a Python script
which in turn would be able to use sys.executable? Python
must be installed anyway.

2nd possibility (probably the best; works for me): in the
.bat files, rely on the Windows feature which automatically
calls the interpreter for .py files, and possibly use 'if
errorlevel ...' to display an error message with
instructions for the user to insert the full path to the
interpreter himself.

3rd possibility: omit the .bat files, and instruct users to
add .py to the PATHEXT environment variable (which might be
difficult for those who don't have administration rights on
their machine), or let the installer apply this change

4th possibility: a binary installer should be able to query
the registry for the location of the interpreter. I just
seeked for python.exe myself and found:

[HKEY_CLASSES_ROOT\Applications\python.exe\shell\open\command]
@="\"C:\\Programme\\Python\\Python24\\python.exe\" \"%1\" %*"
[HKEY_CLASSES_ROOT\Python.CompiledFile\shell\open\command]
@="\"C:\\Programme\\Python\\Python24\\python.exe\" \"%1\" %*"
[HKEY_CLASSES_ROOT\Python.File\shell\open\command]
@="\"C:\\Programme\\Python\\Python24\\python.exe\" \"%1\" %*"
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.py\OpenWithList]
"a"="python.exe"
"MRUList"="cba"
"b"="gvim.exe"
[HKEY_CURRENT_USER\Software\Microsoft\Windows\ShellNoRoam\MUICache]
"C:\\Programme\\Python\\Python24\\python.exe"="python"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\python.exe\shell\open\command]
@="\"C:\\Programme\\Python\\Python24\\python.exe\" \"%1\" %*"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App
Paths\Python.exe]
@="C:\\Programme\\Python\\Python24\\Python.exe"
(...)

The last two might be the most useful, but I'm not sure
whether they'd be present e.g. in an ActiveState
installation as well.