My name is Jessica and I'm a photographer from Brazil. I had a white macbook, and now just got an iMac, and your solution would be perfect to seamlessly walk between both computers without loosing data. With one simple problem, I don't have a MacMini lying around, nor U$1000 to spare ( nor even the Mac OS X Server license ).
I would gladly tho, pay apple for a "family-pack" solution ( like, 5 user ) license to install the Open Directory server alone on any MacOS machine, but they don't have that option. I was a software engeneer on a past life before switching to Photography, but trust me, I'd go for your original solution if I had the cash to spare, I love simpler things :)
But, the iMac replaced an old windows (Pentium IV) box here at home. You think it's doable to just install Darwin/x86 on that box, and use it with the sole purpose of being an small home-based Open Directory Server, with the mobile features you described?