Access to the MTD devices from disk-based OS’es was blocked in the 0.8.x firmware series, see this post for the reasons.
The 0.9.x series does not have this limitation.
I have also done some work enabling “capsule on disk” updates which means the “standard” distribution mechanisms (like fwupd) will work. I think I’ll wait until U-Boot 2024.10 is out before releasing it.