I've kind of found a way to do this:
1) Create a client config called DEFAULT. This is parsed by OpenVPN when a CN is not matched elsewhere.
2) Click the "Blocked" option in the config.
What I'm not sure of is the downside of doing this… The blocked option specifically says that the option shouldn't be used "due to key or password compromise", which seems to imply that it has weaknesses the a CRL does not.
Any thoughts?