Very optimistic about him. I believe out of all the Assist Trophies he stands the best chance of being playable. Yes, all of them. Trust me I have reasoning for this. It comes from Sakurai himself, or more specifically the DOJO.
What needs to be understood is these requests were from Japanese gamers, not Western. Considering the obscure nature of Saki at the time of deciding upon the roster he probably opted to not include him for that reason. Saki Amamiya was likely decided to be an Assist Trophy before knowing that Sin and Punishment was decided to be released on the Virtual Console outside of Japan. Now things have changed for both Saki and the Sin and Punishment series. Thanks to his appearance in Brawl Saki is going to receive double the support he had before, because there will be a segment of Western gamers, such as myself, who are now going to be voting for him. I doubt many of the people who supported him pre-Brawl are going to drop that support; so I see him gaining in that area. While not the best selling title on the Wii Sin and Punishment 2: Star Successor has been released which gives Sin and Punishment an element of relevancy and more material to work with for music, stages, etc. The only issue Saki may face is the idea that it would be better to include Isa Jo over Saki due to him being "fresher". Since Sakurai does not typically work this way, and Isa Jo does not have the support that his father does, I think Saki can shake that issue. Some people have objected to Saki due to his series not selling well, but I don't think Sakurai really looks intently on sales. The Virtual Console re-release sold well enough for Nintendo to create a sequel. I think all Sakurai is going to look at is the fact there even was a sequel.
All in all, Saki I feel stands the best chance. Those are most all of my reasoning.