Which cache directory will flathub allow ensure-writable build-option?

I tried compiling a Gramps flatpak to check the feasibility of adding LiteLLM for an optional addon with AI integration. A looping error in terminal after using the flatpak run org.gramps_project.Gramps command is pasted below:

 Fontconfig error: No writable cache directories
	/usr/lib/fontconfig/cache
	/app/cache/fontconfig
	/.cache/fontconfig
	/run/host/fonts-cache
	/run/host/user-fonts-cache

I believe I can resolve this error by adding something like this:

build-options:
  ensure-writable:
    - /lib/fontconfig/cache

Now this is my question, would flathub allow a flatpak with this permission? Is there a better solution I could add, which would not cause a problem with approval by flathub reviewers?