According to the Rules, Dudson would not be available for the Semi Final as Dual registered players must have played in a group game or the quarter finals in order to play in the semi or final. There is nothing in the rules that says special dispensation can be made so I would assume he cannot play.
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Rule B5:2 (d)
For 2011 only Players correctly registered on the Friday immediately prior to the weekend scheduled for the semi-finals are eligible to play in the semi finals or finals of the competition (where a game is played prior to the weekend ie Thursday or another mid week night then for that game only players must be registered prior to the game concerned). =#0000BFIn the case of Dual Registered Players only players for whom a Dual Registration has been registered prior to the deadline in this B5:2 (d) and who has played in at least one group stage match or the quarter finals shall be eligible to play for the Club in question.
Read rules here: [url=http://rfllive.dyndns.org/~rflmedia/docs/rfl%20section%20B.pdfSection B[/url'"