|Firstly I would like to say that creating a text editor with plain batch is an uphill battle with a broken leg.|
Be aware that some oem codepages don't seem to include line drawing characters at all.
The trick with getting those line drawing characters into the script is to get the character that corresponds the same ASCII(ANSI really) value in the gui environment copy it into the script.
Copy the list from #7 in this thread to a text file and run TYPE on it to see what the characters look like, they may even be the ones your after. It may work on other codepages than 850.
Another thing that is an issue in xp(don't know about latter) is that some fonts continue to display the default oem code page even after using chcp.