Please give us an eselect xsession module for setting /etc/env.d/90xsession.
/etc/env.d/90xsession used to be owned by x11-apps/xinit. Nothing owns it now so it's hard to tell why an eselect module should write to it in the first place (since no package mainatains the file) and where it would source possible values for XSESSION.
AFAIK it's used for setting the system wide WM for users not having ~/.xinitrc Values are in /etc/X11/Sessions/