Let me know if I'm wrong, but I think you're mixing up a few different questions. One question is, "how do I use my meter and will incident or spot work better for that?" The other question is, "how do I use best use compensating development with roll film?" You'll find many threads that deal with each of these questions if you search prior posts, but maybe this will help a little:
For the first question, I (and probably most B&W zone system users shooting negative film) use a meter initially to assess the entire contrast range of an exposure, then to set exposure to place a particularly shade where I believe it should be, and then to assess where highlights will fall based on that exposure setting. If you're shooting mostly landscapes, I think a spotmeter is more useful in the second and third parts of this process as you don't have to walk out to the subject area (e.g. a cave on the other side of an inconvenient canyon). If you're shooting mostly portraits, then this is not a problem since you only have to walk a few feet.
You asked "how to take advantage of a spot meter when using roll film." Well it's no different than with sheet film. The meter is used to help calculate and set the exposure that will produce the image you have visualized. That exposure will be whatever it is, regardless of what type of meter you're using. You just need to figure out how to use the meter that you have to obtain the specific data you need to calculate that exposure. So your decision on what type of meter to buy should be guided by the process you see yourself using it in.
As to the second question I wrote above (about compensating development), that's probably what you're more interested in dealing with. As you said, shoot two cameras or one camera with two backs (one for high contrast situations and one for low contrast). If you're going to use compensating development I don't believe there is an easier way and I don't think using two backs is that cumbersome. Again, what kind of meter you use doesn't impact this question much, aside from your being able to initially determine the full contrast range of your subject, selecting which back to use and noting how much compensation to use in development.
Sorry for the longwinded answer, but I hope it helps. To sum it up, here's a landscape example. Shooting a frontlit mountain range from the valley in early morning light, I set up my composition and spotmeter the darkest area in the scene where I want detail. I calculate exposure so that the area is Zone 3 or 4, then I spotmeter the highlight areas to see where they fall. If they are over Zone 7 or 8, I use the high contrast back. If they are not over 7 or 8, I use the low contrast back. I then note the exposure settings, the shadow and highlight points and the compensation I intend to develop with (I carry a notebook for this always). Shoot. Develop the roll appropriately (which will be a trade-off based on the range of compensation needed for the shots on the roll and the relative importance of each of those photos).
