Since Toolbx mounts are running in different namespaces than the host, the mounts cannot be accessed from the latter. I tried many workarounds to see if it really was possible to get around this limit. After some time I decided that using the network stack was probably a better solution to communicate between Toolbx and host.
In my particular case I never do any layering but instead try to use Flatpak, Toolbx, or even a Boxes virtual machine for any specific software not included in the base layering.
I needed to run restic to access some documents on a remote restic repo backup. Since restic is not included I had to use Toolbx for this task. Restic uses FUSE to mount the remote end on the local system. In an ideal world I could just have opened this mount in Dolphin to search and open the needed documents. However due to the namespaces difference this mount just shows empty on the host.
A really quick and dirty solution is to simply use rclone and its webdav option, then Dolphin just opens the webdav address with no issues. Yes, there is a tiny performance hit, but the speed and easy execution greatly makes up for this:restic mount ~/remote-restic
Now that the mount is working from Toolbx itself run rclone to serve it over the network:
rclone serve webdav ~/remote-restic --addr :8080 -L
That is all! Now simply open Dolphin and paste the webdav address and there you have your Toolbx mount visible from the host:dav://127.0.0.1:8080
Please note that this is not only for restic but can be used with any mount type like sshfs or even another rclone mount like S3.
Is it not ironic… using a 1990s protocol (WebDAV) to solve a 2026 container problem!