Indeed, that work with or without quotes. But I prefer my method, because Control-C doesn't work with yours (due to the /B argument). And anyway, adding the path to the toolset in the %PATH% variable doesn't hurt. I need it anyway.
I still don't understand why the quotes make it fail, especially because it's only a problem with my syntax and not with yours!
And to determine the cmd.exe path, I will probably use another method. I can retrieve it with BD3D2MK3D with the command
where cmd.exe
It is probably also possible to use the ComSpec environment variable. On my machine, %ComSpec% is exactly C:\Windows\system32\cmd.exe
However, I don't know if it can point to another exe on some systems.