You can use decky loader in desktop mode if you launch steam using big picture mode (there is an option for that in the menu). Not all the plugins work outside of game mode though
you seem to know quite a bit more about this than me, is there any potential way to make this work in the long run (the mill dev seems pretty ameniable to changes overall and the lack of other desktop plugin solutions for bazzite might be a big selling point)
I tried a few things. Couldn't get it to work. You might be able to install it in an arch distrobox. But then you'll need to re-install steam in the distrobox too (which completely defeats the point of Bazzite). I also tried putting the files in /usr/local/* instead of /usr and changing the start.sh but it looks like the application has hardcoded paths in it. So you might need to wait for the author to make a flatpak or appimage version or for someone else to make an rpm package.