IMR (in my research)
i found that what you would like to do at this point is not an option. It would take adjustments on the part of W3C and the doctype to accomplish this, and even though IE and firefox (maybe others) have this functionality, making an attribute for a hyperlink to target a new tab, is platform dependent and i dont see it being implemented.
If developers were able to access this "feature" im certain popups and adware would be targeted there too, and i personally dont want that to happen!
until that day, teach your users to ctrl click the link, or somehow emulate the ctrl key being pressed when clicked.
Good luck!