Hello,
I a going to be doing that very thing over this weekend. I have changed over to Silverblue from standard Fedora Workstation, due to the promise of easier development through the flatpak/container model. So far I am not at the point of installing Java, which I assume has to be layered if I want it available to any faltpak. I gave up, temporarily on trying to get Evolution installed as a flatpak, since currently faltpak-builder will not install into the ostree (I am using F28). As I get to the install, I’ll have more info that I can share about using Oracles JDK 8 and 10 on Silverblue. From the info up top it appears that Oracle installer is pointing to the wrong location for basename perhaps. You could try to modify their install script.