Visit my.sonatype.com for documentation on Nexus Repository version 2.
There really is no good way to achieve the same thing using Nexus Repository 2 OSS, this use case is the entire reason that we developed smart proxy technology.
The closest thing you can do is set the metadata max-age in the proxy to zero, this will force Nexus Repository 2 to check the remote for all inbound requests to the proxy.
The problem you will encounter when doing this is that maven-metadata.xml file requests to group repositories require that Nexus Repository 2 checks every member of the group for the file, since it may exist in multiple locations. So all inbound requests to the group for maven-metadata.xml files will end up checking the remote of the proxy that has this cache timeout set to zero.
You can control this somewhat by adding manual routing rules for the proxy repository.