I discovered eventually that using the Archive function does, in fact work - as long as the hostname of the new machine was the same as the old. If the hostname was different (as, in fact, it was when I was trying it, since it was physically a different machine), then when trying to restore on 10.7 in the Server Admin, while it doesn't give any errors it also doesn't work. Once I managed to dig up the command line method, however, I discovered that it was giving an error about the hostname being different - Server Admin simply doesn't bother to show that . Renaming the old machine and changing the hostname of the new one to match what the old one was did allow me to move the directory across.
I'm soon going through a similar move... just shares, DHCP , DNS and OD is relevant. DId you set up the new server as new, change the hostname and then import OD, in that order? all shares reside on external storage, they should retain permissions, right?
Oh, what version of 10.7 did you import to? 10.7.4?