How big is the closet in the spare bedroom? I rented a small 2 bedroom apt. for 6 months. The small 'master' bedroom closet was my office space because I worked from home. A little desk, my laptop, a phone, a nice poster to substitute for a window, all set up next to the open closet door for extra light and to combat claustrophobia worked. The overhead florescent lamp, plus the open door were enough light.
Even so, I could not have used it for a sewing space because of my tendency to spread out my work. It's one thing to type into a computer and quite another to have creative clutter (aka known as a 'mess') all around while you sew!