It's your own code you'd mainly have to worry about.
But, alas, I always seem to get called in to debug code from other folks.
You probably don't get called in to debug endianness issues in any TCP stacks in portable general-purpose non-hobby OSes; if so, I'd look a bit askance at the developer team for that OS.
My point is that worrying about porting server OSes for PPC machines is not something worth worrying about, as they've either already been ported (Linux) or, if they ever get ported, are likely only to be ported to little-endian machines using compilers that aren't going to differ between platforms on the signedness of char (Windows). What you worry about there is third-party applications.