Hi Estel,
well the JAF-Box looks like the way to go. I mean it looks like the Docking Station I was about to build. And if the RJ45-Adapter (the network-thing) supplies enough power and if the flasher recongnizes the n900 via network it should be possible to flash the n900, shouldn't it?
Even if not with the JAF-Box it looks like a easy task, cut off the RJ45, measure what is what, connect USB accordingly, flash my beloved n900, sell my new Android phone

Cheers,
blck