If I were to ask a professor on sabbatical to consult with him on a paper I'm writing, would I be imposing on him?
No, I don't think it's rude at all. 

A sabbatical is not meant as a complete vacation from work. Rather, it's a time for the professor to focus on scholarly work without the distraction of teaching or (usually) administrative duties.

I'd say that asking the professor to comment on your essay is definitely a scholarly activity, and so it's a reasonable request.

Of course, it's up to the professor whether he actually wants to do it or not, or if he has time to do it given his other activities. Don't be offended if he declines. Also keep in mind that many professors travel extensively during sabbaticals, or spend the entire time at another institution, so even if he is interested, he may not be able to meet with you in person - you may have to discuss it over email, Skype, etc.