There are two main choices of how to design the switchmode converter.
1. A single switchmode buck converter that outputs 4.2V. This will then be fed to individual linear current limiters for each cell.
2. A seperate swichmode converter for each battery. To make things simpler, I'll probaly use an intergated switchmode controller that only needs an external inductor and filter caps. Such as the MAX17504 (yes they're available as free samples) which has an input of 4.5 to 60V and has a synchronous rectifier.
Originally I was going with option 1 but I've since changed to option 2 instead.