There's got to be a better way than copying the /etc/oraInst.loc every time we install to a different $ORACLE_HOME.
/etc/oraInst.loc? What version of Oracle are you using that puts a file in the /etc directory? I've only seen /var/opt/oracle, created by running /tmp/orainstRoot.sh if /var/opt/oracle doesn't already exist.

Once the oraInst.loc file is created, OUI looks for the location specified in it, then reads the oraInventory stuff. You can have multiple Oracle products installed in their respective ORACLE_HOMEs, with all of that information stored in the original oraInventory directory if you let Oracle do what it wants to do.

The only time I've had to dummy up the oraInst.loc file is when getting abused by the iAS/iDS installation process, and that was going off the reservation for other reasons (needed to do that so iDS would install using the JRE directory created with the iDS install, and not using the JRE directory from a previous Oracle product install).