OK, with some further research, here's the workflow I've devised that I think will do the job:
1. Invert the DEM by multiplying by -1
2. Follow the steps in the help system's hydrologic analysis sample applications topic to produce a watershed analysis.
3. Convert the watershed to polygons and select those containing my ridgelines.
4. Use this new layer as a mask and continue with the original set of instructions to derive my buffers.
Next up, understanding pour points and how they affect the watershed.