You're describing soreness in brachioradialis, not the elbow conditions like golfer's or tennis elbow, so any stretches need to be for that. I'm not aware of any dumbell eccentrics for this muscle...have you seen a physio?
Should it go into the wiki? I've posted it enough times!
http://www.athlon.com.au/articles/r&i_dodgyelbow.pdfthis article mentions brachioradialis pain
and suggested eccentric dumbell exercices.