It's been done before, of course-- so you could do it as well.
I'd hold off buying the parts on the web or even new. Not all barends are the size-- you'll need to find a pair that works. I'd look for someplace with a whole lot of used ones-- then you could try for the right fit.
Also take a look at Paul's Thumbies-- a great way to mount downtube shifters on the handlebars.