I dont know what squeezelight is .. everything is possible, but that would probably require a recompilation of squeezlite and then how would you have it cooperate with rasplex.
In terms of architecture, i'd tend to think that it would be better to have PMS find and eventually list squeezebox stuff, that's be more in the Plex Picture.
Maybe you could ask this on their forums and see if they would look into it :)
Some quick googling shows that a Squeezelite implementation is available for the RPi under the Raspbian Linux distro.
This means that you could make a 'BerryBoot' installation offering the choice of booting either into RasPlex or into Raspbian (to use Squeezelite).
(At each boot BerryBoot displays a menu where you can choose which firmware to run that time.)
But there would be no quick way to jump between the two clients, as you'd have to make a full reboot to switch between them.
Also, running any RPi firmware through BerryBoot will be slower than running it standalone, due to the virtual filesystems used by BerryBoot to hold each firmware.