im not too well versed in flatpak, but just spitballing here, off the top of my head, i could totally be wrong and LD overrides just applies to regular executables running outside a sandbox
With the flatpak versions of obs an gstreamervaapi plugin i am only seeing the legacy option. My flatpak list | grep obs output: OBS Studio com.obsproject.Studio 29.1.3 stable system OBS VkCapture ...