Despite what SDDM maintainers said it seems that the code itself does[1] (or, at least, should) source /etc/profile and $HOME/.profile, provided they exist[2]. I am also pretty sure that Kubuntu capitalizes on this functionality to handle nvidia-prime.
[1] https://github.com/sddm/sddm/blob/master/CMakeLists.txt#L150 [2] https://github.com/sddm/sddm/blob/master/data/scripts/Xsession
