The radios are tuned by software command; they're not tuned by internally fixed constants (e.g., firmware configuration).
If your software tunes a 915Mhz product to 868MHz, the penalty will be small, about 10%, so I've read.
great demo and great progress. Can I ask when are you planning to let out a beta version? I would load it up on my config - I have 2 x miniwireless with RFM69HW and 4 x Arduino pro mini with RFM69HW (breadboard and pt to pt wiring(!) but no RTC or FLASH).