"Cannot find a Server Preferences.app that runs on a 10.7 client to let me edit accounts on a 10.6 server."
That's because you can't use the Server Preferences.app on a 10.6 Server. If you want to do this from a 10.7 client then you should vnc onto the server and using WorkGroup Manager and/or Server Admin that way instead.
Go Menu > Connect to Server > vnc://IP address of your server.
At the prompt provide the server's admin name and password. This assumes you've enabled Screen Sharing or Remote Management. Both options are available in the Sharing Preference Pane.
WorkGroup Manager and Server Admin are still available as a free download from here:
You can install these on a 10.7 client. Launch WorkGroup Manager and connect to your server and manage/edit accounts that way. Changes are written up to the Server. Similar situation for Server Admin.
However I would be careful about doing it this way as depending on which services you are running, you may get unexpected results especially if your 10.6 Server is a mature one. You should really try and match server, client OS as much as possible with the correct versions of the server tools.