Ability to use a custom Java binary

Due to a problem with Forge when using Java 8 >= 8u312 with 1.16+ modpacks, I am unable to play [this modpack](https://www.curseforge.com/minecraft/modpacks/vault-hunters-official-modpack). The problem is better described [here](https://github.com/MultiMC/Launcher/issues/4470).

To resolve the issue, I have tried downgrading Java, but quickly realized that the Java versions are baked into the flatpak and cannot be changed easily. I was not able to get the flatpak version of PolyMC to use a custom Java version from my machine.

Additionally, I have tried toying with flatpak extensions (namely [this one](https://github.com/flathub/org.freedesktop.Sdk.Extension.openjdk8)), but that lead nowhere.

Is there a way to use a custom Java binary with the flatpak version of PolyMC?