Well, I don't have any solid rumors, but looking at the way you're thinking.... maybe Parys Haralson? He isn't amazing, but he had 8 sacks on the year, played in a 3-4.
A real longshot would be signing a really old guy, but I really don't see this happening. Someone like Colvin, but he hasn't done anything this year and couldn't pass a psysichal earlier on. Another one is Clark Haggans. He had just a one year deal with the Cards. He hasn't had an amazing amount of sacks either, is 32 years old an has barely played for the Cards this year.
We'll see who Ted Thompson will get. For all we know he is indeed happy with his current roster and won't sign anyone. Didn't Poppinga line up as a DE from time to time? And isn't Jeremy Thompson switching the OLB aswell? Maybe we'll get a rookie aswell and in that case, I think we're pretty much set.