Basically however you slice it, you need a way to have two operating systems installed. Whether they're squirreled away in a BTRFS snapshot or unpacked as hard links is really an implementation detail.
The *real* challenges are around configuration management, how you present this to the system administrator, etc. For example, it's clearly desirable if the system software management tool can tell you stuff like "you have N OS images installed, the additional snapshots are taking up XX megabytes of disk space compared to the current".
To do that kind of thing, you need something aware of *both* the rpm/deb state and the BTRFS state.
Furthermore, one huge advantage of the current OSTree design (that I need to promote more on the wiki) is that it parallel installs inside your existing distribution - you don't need to switch disk formats. All you sacrifice is disk space.
You don't have access to that package of bioinformatics software in the latest GNOME OS? No problem, reboot into your regular host distribution and use it. Switch back when you want to see the latest GNOME.