When stopping the server and restarting it the ppc errors or drops out to the win CE screen. I found a few posts where it's mentioned and it might be fairly simple to address. It would also tie in well with using wake on lan sleep mode for the htpc that is the server.
When the PPC fails to see the server, instead of the error window that is now used, display a screen that is saved on the client (maybe just one skin page that is downloaded and saved in the directory on the ppc where xlobby lives). The screen would have a button to exit and one to wake up the htpc, which would be a simple ping command directed at the server address - I guess doing it this way you could define whatever buttons and have a field in XL that chooses the 'offline ppc skin'.
Of course any other communication issues that might cause the client to be unhappy prior to talking to the server (if any) would have to be addressed.
There's some discussion of it starting with sdumas's post almost halfway down...
http://www.xlobby.com/forum/viewtopic.php?p=9035&highlight=colby+ppc#9035