@Harry: I'm not sure what service you're thinking of, unless this was an ironic way of referring to MathSciNet, but I really doubt any online list will make it clear who has had some thought on a problem a few years ago and moved on, and who is continually returning to it. The latter information tends to spread through the mathematical community by word of mouth, so MO might be able to do better.
@Sean I think you should ask about one question at a time. For example: "Which mathematicians are actively working on quadratic generation of toric ideals of matroid polytopes?" strikes me as a fine title for an MO question.
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