Corey,
I'm running focal/usrruri/octopus
There may be something that I'm missing here.
sudo add-apt-repository cloud-archive:ussuri-proposed
 Ubuntu Cloud Archive for OpenStack Ussuri [proposed]
 More info: https://wiki.ubuntu.com/OpenStack/CloudArchive
Press [ENTER] to continue or Ctrl-c to cancel adding it.

cloud-archive for Ussuri-proposed only supported on bionic

from reading through the docs on testing proposed, it appears the way to do this on focal would be
cat <<EOF | sudo tee -a /etc/apt/sources.list.d/ubuntu-$(lsb_release -cs)-proposed.list
# Enable Ubuntu proposed archive

deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe
EOF
cat <<EOF | sudo tee -a /etc/apt/preferences.d/proposed-updates
# Configure apt to allow selective installs of packages from proposed
Package: *
Pin: release a=$(lsb_release -cs)-proposed
Pin-Priority: 400
EOF
  sudo apt-get update
# verify that proposed updated will only be installed when specifically requested
  sudo apt-get upgrade -s 
  sudo apt-get install os-brick-common/focal-proposed

which appears that it would uninstall cinder

Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '3.0.1-0ubuntu1.3' (Ubuntu:20.04/focal-proposed [all]) for 'os-brick-common'
The following packages were automatically installed and are no longer required:
  alembic docutils-common libblas3 libconfig-general-perl libfreetype6 libgfortran5 libimagequant0 libjbig0
  libjpeg-turbo8 libjpeg8 libjs-sphinxdoc libjs-underscore liblapack3 liblcms2-2 libpaper-utils libpaper1 libpq5
  libtiff5 libwebp6 libwebpdemux2 libwebpmux3 pycadf-common python-babel-localedata python-pastedeploy-tpl
  python3-alembic python3-amqp python3-anyjson python3-automaton python3-babel python3-barbicanclient python3-bcrypt
  python3-cachetools python3-castellan python3-cffi python3-cliff python3-cmd2 python3-cursive python3-ddt
  python3-debtcollector python3-decorator python3-defusedxml python3-docutils python3-dogpile.cache python3-eventlet
  python3-extras python3-fasteners python3-fixtures python3-futurist python3-glanceclient python3-greenlet
  python3-iso8601 python3-kazoo python3-keystoneauth1 python3-keystoneclient python3-keystonemiddleware
  python3-kombu python3-mako python3-migrate python3-mimeparse python3-monotonic python3-msgpack python3-networkx
  python3-novaclient python3-numpy python3-olefile python3-os-service-types python3-os-win python3-oslo.cache
  python3-oslo.concurrency python3-oslo.config python3-oslo.context python3-oslo.db python3-oslo.i18n
  python3-oslo.log python3-oslo.messaging python3-oslo.middleware python3-oslo.policy python3-oslo.privsep
  python3-oslo.reports python3-oslo.rootwrap python3-oslo.serialization python3-oslo.service
  python3-oslo.upgradecheck python3-oslo.utils python3-oslo.versionedobjects python3-oslo.vmware python3-osprofiler
  python3-paramiko python3-paste python3-pastedeploy python3-pastescript python3-pil python3-ply python3-psycopg2
  python3-pycadf python3-pycparser python3-pygments python3-pyinotify python3-pymemcache python3-pymysql
  python3-pyparsing python3-pyperclip python3-redis python3-repoze.lru python3-retrying python3-rfc3986
  python3-roman python3-routes python3-sqlalchemy python3-sqlalchemy-ext python3-sqlparse python3-statsd
  python3-stevedore python3-suds python3-swiftclient python3-tabulate python3-taskflow python3-tempita
  python3-tenacity python3-testscenarios python3-testtools python3-tooz python3-vine python3-voluptuous
  python3-warlock python3-wcwidth python3-webob python3-wrapt python3-zake sgml-base tgt xml-core
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  cinder-api cinder-backup cinder-scheduler cinder-volume python3-cinder python3-os-brick
The following packages will be upgraded:
  os-brick-common
1 upgraded, 0 newly installed, 6 to remove and 0 not upgraded.
Need to get 3440 B of archives.
After this operation, 25.5 MB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.

Is there something that should be done differently in order to test the proposed fix?