Does anyone think it might appear to be a conflict of interest to the professor or insulting? I had wanted to do something similar, especially for the letter writers who are not only writing me a thousand letters individualized to each institution (i.e., not from a template) but also have mentored me through this process, fielded reference calls for jobs, gotten me published, etc., going far beyond a simple 5-minute fill-in-the-blank LOR. I would like to be able to return those favors, but I recently saw (on another forum) where a professor actually said she would consider it "insulting" because she might feel like the person was paying her off to do her job and serve her students.
It was an interesting take...