John, if the leather is veg tan, you may have to adjust tolerances for the pipe since the leather will not be as supple as chrome tan. However, I think it will look better and I would approach it like this, Since the pipe is the main feature, I'd build my pattern around the pipe, and I'd wet form the case around the pipe for a custom fit, then let the rest of the pouch fall into place based on that. I think there are lots of cool ways you could go about this.
If you are using another pouch as a pattern, I'd suggest mocking one up out of paper or thin cardboard first.
Hope this helps.