I talked with one of the maintainers of central (Brian from Sonatype) about doing this, and he is open to the idea. The first thing we need to do is allow rsync from our repository. In the past, operations has not been very open to allowing this. I explained the benefits related to reduced load on our server and they are more open to the idea now.
Once rsync is set up, the plan will be to sync specific groupIds from our repository. The artifacts going to central will have to be validated first (to check for invalid POMs, incomplete dependency tree, etc). So longer term we should be able to set up the same validation rules in our repository that are used for oss.sonatype.org and for central, and then eventually sync everything or at least the majority of our stuff.