Neat hack. I am slowly building up an arsenal of 5.5mm to 4.5mm adapters.
As it happens the choice for 4.5mm power connector was ultimately driven by the board-to-board separation between the base board and the mezzanine (which is itself driven by the physical specs of the mezzanine connectors). A more conventional 5.5mm barrel is simply too tall and would have required mezzanine designers to put a cut-out in their board to work around the jack.
One of the ambitious parts of 96Boards CE spec is the small physical form factor (even two years later it remains challenging to route the boards).
Personally I like having tiny, tiny development boards so gladly tolerate the unusual power jack (and lack of RJ-45). However, let's just say I know that plenty of people who would and have advocated for a different compromise...