Thanks for this, dlanor, even if it's not what I wanted to hear :-). BerryBoot is no option for me as the final solution needs to be wife-proof. To have both Rasplex and the Squeezelite client running at the same time seems like an extremely reasonable request.
That depends on how you define 'reasonable'.
OpenELEC is an operating system optimized for running a single main application, plus whatever background services it needs.
It's not designed for running multitasked applications, or even for switching between any applications.
(It's designed to autostart its single application whenever that is terminated, except for shutdown cases.)
So running other applications in parallel to RasPlex under OpenELEC is not reasonable seen from how that OS is intended to be used.
A more reasonable approach would be to link the new client behavior into RasPlex somehow, as done for XBMC in the thread you linked to.
Using autostart.sh did not work for me, http://openelec.tv/forum/124-raspberry-pi/74692-squeezelite-on-rpi-unable-to-play-mp3-files#131047
An XBMC Hack to load one's own libraries doesn't work for Plex and seems to go a bit too far (replacing existing libraries instead of adding them), http://forum.kodi.tv/showthread.php?tid=162307&pid=1481392#pid1481392
PHT shares some ancestry with XBMC, but they have both evolved further in different directions since that project branching, so there are many ways in which they are no longer compatible.
The method described in that linked forum thread assumes that the XBMC style library system is intact, and for PHT those parts have been rewritten to deal with the PMS library system instead.
It's possible (even likely) that some squeezelite method may be developed with PHT compatibility, but I don't think it can be done by simply using the binaries made for XBMC.
Also, I think this kind of extension of PHT abilities is something that should be proposed for and implemented in the official PHT project, whereafter it would also be ported to unofficial implementations of PHT like RasPlex.
Best regards: dlanor