Are you doing this for an RPM package?
Yes, and the spec file created an on the fly entry for the wmsession.d directory. That was the thinking to post here. On second thought this may have been better posted in the advanced section. Or on third thought, this post can be deleted. It's probably not going to help anybody.
And yes, this was used to create an rpm package for a window manager that is not in the repository.
Doing this did bring up a question. What is difference between creating an entry in wmsession.d (and then using %make_session), or putting an entry in /usr/share/xsessions?