Yes, and realistically the fact that the whole drive needs to be dedicated to the HR20 will probably never change. But I can definitely see them setting it up so that at least all settings are stored on the internal drive. It would be great to have both drives be available for recordings too.
This shouldn't be a problem. My Cablevision SA 8300HDDVR can do this, all you need to do is unplug the power, plug in the eSATA, reboot and 5 minutes later you will have 2 drives connected together (internal and external) and you can record on both.
The Box decides which drive to record on, usually the one with more space available.
So this shouldnt be a major thing for Directv to activate this on Our HR20-700's