I have some experience with RNAi using pHannibal.
About 50% of plants containing a 700 Bp inverted repeat directed to the
3' end of the CH-42 gene show some chlorophyl deficiency (CH-42 codes a
sub-unit of Magnesum chelatase and is needed for chlorophyl
production). I think you need to contact people who have a lot of
experience using hairpin constructs you might try Agrikola
(http://www.agrikola.org/) or Chromodb (http://www.chromdb.org/).
Kind Regards Ian Furner