Trying to do this could get real complicated real fast.
The first issue is that there are multiple root CAs. I think it's
been over a year now since they started using the newest CA, so within
another year or so there shouldn't be anyone left with certificates
based on the previous root CA.
The other problem is that end-user certs aren't signed directly by the
root CA like OpenVPN expects. There may be a way to get OpenVPN to
trust them if the intermediate CA cert was included. However, OpenVPN
doesn't natively support specifying a cert chain. It may still be
possible by concatenating the end-user cert with the intermediate cert
into the same file, but that will require testing to be sure. That
and trying to come up with easy to follow instructions for your
clients to accomplish this could be a challenge too.
I agree that it's a worthwhile project, but I just wanted to point out
the challenges real quick. Hopefully, I'll have more time to help
figure it out someday. ;)