Is it possible to write a senior essay in a lecture course instead of in a seminar?

No, the seminar requirement is one of the Department’s hard-and-fast rules. If you would like to work with the professor teaching the lecture course, you might approach her or him about advising you in PLSC 480 while you audit the course. Or you could still enroll in the lecture course, but in that case, any written work you produced for it would have to be substantially different from your senior essay.