Hi.....sorry, it does have to be isolated.....i would have thought it would have been safe enough with non-isolated-but-transformer-based SMPS's, but no, we need isolation of some 500v or so...just in case the 100V battery somehow gets through to the end point of the circuit and zaps someone.
I suggested Vicor micro and PRM/DCM ranges, but they are thinking vicor is too pricey.....but in my experience, this spec has got vicor written all over it, from a size, cost and efficiency point of view...would you agree?