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? I'm a fourth year undergraduate philosophy student. 

Last academic-year, I took a seminar with a professor that I had known reasonably well before I took the course. During that seminar we had several lengthy after-class conversations about a topic related to the seminar. The professor seemed to enjoy the conversations, and I think we developed a good relationship (as far as undergraduate student-professor relationships go). 

I plan to write an essay and submit it to an undergraduate journal in December. I know that the topic of the essay is interesting to the professor and covers some unconsidered possibilities that the professor expressed interest in during our conversations. That is to say, I think he'd enjoy thinking about what I plan to write about. 

This professor is the only one in our department knowledgeable about the subject, so I can't ask anyone else for their thoughts on the essay. Unfortunately, he's on sabbatical this year. 

Would it be rude to email the professor in order to ask him whether I might discuss my argument with him a few times during this term?
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.