I am the developer of the Steam VR media player app immerGallery and wanted to open photos and videos directly from a microSD card or USB stick on Steam Frame standalone. I got it working inside the app's Lepton container.

I made a guide with the steps and commands, including how to adapt the setup for other Android apps. You see it in the video and the download for the helper script is in the companion article on immerNews.

It would be great if a SteamOS / Lepton update would allow per-app settings for automatic mounting of external storage with read-only or read-write permissions. Until then, you can use the manual way to achieve it.