Will consider this.
Note that the latest version of WinSCP has Continue on error option:
Looking forward to this feature!
I think you'll be the first to have this type of "don't touch open Windows files" feature. Think about all the people creating all these crazy workarounds (using temporary extensions, etc) because no SFTP app can just check the "file is open" state.
btw I have been trying and noticed the "continue on error" issue is pretty aggressive. It actually somehow is causing other Windows programs to fail just from WinSCP "touching" the file, trying to copy it. Probably a Windows thing, but WinSCP tries like every 5 seconds over and over, which somehow causes problems in the apps that have the file open