Haven't tried it, but the documentation suggests that DefineBitsLossless2 might be the right class: 

Tag::DefineBitsLossless / Tag::DefineBitsLossless2

     defines a loss-less bitmap. DefineBitsLossless2 contains alpha channels

In reply to Re: Perserving transparency png into swf by almut
in thread Perserving transparency png into swf by Anonymous Monk

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