I've worked some more, with this as a start.
I've decided to make a buck with transformer isolation after I've finished my boost - psu charger. This is the fast scetch I've made on this circuit: (It's missing a lot of resistors, capacitors and ground symbols)
It's duty cycle will look something like this:
Imo this could be a fast and safe charger. The transformer is a 1:1 ratio. It's just there to provide isolation. The switching and deadtime is controlled by a microprocessor. It will use mosfets capable of withstanding 400vdc, the positive is that its load is more or less continous (less EMI) - the negative is increased losses since these mosfets has a bit higher on-resistance. A current sensor is utilized to sense the current - and adjust dutycycle accordingly.
If anyone would like to start, I could help out with the avr software, given that you prototype it with a xplained 1284p AVR (these are cheap and nice to prototype with).