I don't think it is feasible with a plain textarea. You may want to use a Javascript-based replacement for textarea (with all the possible browser issues it may bring).
Here's one that I found out (but didn't try). With it, it will allow the writer to justify the text amongst other things.
http://www.codeproject.com/jscript/htmlArea.asp
From there, perhaps you could search for a Javascript that strictly does justify formatting... You didn't specify why you needed that though - if it's from a GUI perspective, you might want to do your form in Flash, although I can't bear those.
Entreri.