I am more or less where you are. I have used sculptor, siteassist and menuwriter for 2 years and had nothing but problems. I bought updates, hoping they would fix issues just to find out that they did not. Out of desperation, I completely stopped using the extensions and uninstalled sculptor and siteassist, as they would constantly break my designs, clutter my harddrive with random temporary files and cost me days of work to undo the damage just to find that next time I ran the programs, they would mess up again. The only "stable" and usable webassist extension I use is menuwriter despite its ridiculous "target" issue. I am wondering whether the developers actually design websites, considering those basic shortcomings.
HOWEVER, I am wondering whether one might solve the problem enclosing the menu in a <div id=navigation> and then defining a style for external links with :before pseudoelement combined with a  (backspace) in order to rewrite the opening part of the statement
content:  target=blank >
Unfortunately, the content property only supports text strings.
Anybody can think of a way of doing this short of Webassist getting their wits together and posting a service update including "target"?