I have heard of people who have taken a tablespoon of flax oil a day and have had their mild cases of psoriasis totally get better. As you probably know everyone has a different psoriasis trigger and maybe flax works well to resolve the triggers for some people but not for everyone. Same thing with zinc suplements.
So, maybe you can try flax and see whether it works for you. I think you need to try it for a long time (maybe 8 weeks) before you see any improvement. I'm still taking it and it's helped me a bit, but not cured me.
As far as what form of flax you could have, I don't think you get enough flax in a muffin or a cereal to make a difference. Flax meal with honey might be another way to go (take some flax and grind it and add some honey) but you'd have to take, oh, 1/2 cup? Something like that. It's a lot of flax meal to get down. I think that oil is the only way to go. Of course, you can still have the other foods as they provide much fiber which is really good for you otherwise.