In emacs23 -q,

    C-h f post-mode

doesn't have a link button to the post.el source code, where I hoped
it would.

In emacs23 a .el must be in the same directory as the .elc to be
recognised as its source code.  The easiest way to do that is for the
install script to symlink the .el files to the flavour directory when
byte compiling, and leave them there for the help system to use
(removing both .el and the .elc in the remove script).