Another short answer is yes. It is possible to do an early walk-in at a separate facility than listed in the biometrics appointment notice, but it's not guaranteed. And it wont affect the timing of your processing.
As with many processes in USCIS, unfortunately it depends on the day and mood of the USCIS employee (in addition to schedule constraints). I suggest you try an early walk-in somewhere in the Eastern US. To accomplish this 1) I would research on here when are the better days to try 2)when at facility be clear, nice, and well organized/justified/presented (e.g. bring your plane tix showing you'll be in Eastern US during scheduled biometrics appointment), and it may just work out for you.
See thread: 'Re-Entry Permit: Applied After Departure' for a success story that did just what your contemplating.
