Thanks for working on it, but I think we'd prefer a solution that doesn't require the maintainer having to think about obsolete platforms that lack fchdir.
How about the following idea instead? On platforms lacking fchdir, put a wrapper around 'open' so that we can keep track of which file descriptors correspond to directories. The 'open' wrapper puts the name of the opened directory into a hash table. (The name must be absolute, so 'open' may need to do the equivalent of pwd. This could be further optimized by wrapping 'chdir' and 'close', though I'm not sure it's worth the trouble. We might also get away with using O_DIRECTORY to decide whether to put an entry in the hash table.) Once this wrapper is in place, we can implement 'fchdir' as a function that looks up the file descriptor in the table, and invokes chdir on the resulting file name. (The code also must check that the file descriptor is still open, if 'close' is not wrapped.) The emulation wouldn't be perfect, but it would be good enough, and it could be isolated into one small region of code (presumably a new Gnulib module 'fchdir'), such that we wouldn't need to worry about this problem in the rest of coreutils.
