Flatpak includes bubblewrap as a git submodule and builds itself a
little hybrid helper program.
I should note that ostree itself requires glib (with or without
introspection), and beyond glib, the introspection data for ostree and
pygobject itself there are no other GNOME specific dependencies (in
other words, I would hardly call this "pulling in the whole GNOME
stack").
I'm not sure what to do for RHEL 7, if they are adamant about not
including an suid bubblewrap (or even a non-suid bubblewrap) then I
suppose we could try looking into a linux-user-chroot solution (not
sure, is it worth going this far for RHEL 7 ?).
Note that some distros have bubblewrap non-suid and instead require
that the OS allow regular users to create namespaces and drop
priviledges.