Constantin:
I would focus on the end power needed. In the case of a server, you're typically looking at 12VDC for the HDD's and 5VDC for the board. PicoUPS .....that plug directly into existing motherboards.
THAT'S really one thing I compleetly missed to think about. Thanks a lot! Just one question: aren't there +12V and -12V for some boards (aka a diffrence of 24V)?
But I like the idea a lot. Use a good PSU as primary converter (which has the additional capability to smoothen the Inputpower) and work with PicoPSU the rest of the way. Only concern to me is the speed of switching to the batteries in case of powerloss. Using some big capacitors for the switching delay, perhaps?