Either of those would work fine, but this still doesn't break any ground on the following two questions.
1. Is there any existing software that will will interface with an RTC via I2C? As said previously, it would be simple enough to add something to init.d for startup and shutdown... but what about power failures? We need something that will keep the RTC in sync, etc.
2. I'd also like to know if the CPU itself has an option for RTC battery backup. (Even if we can't use it)
EDIT: found the answer to question (1) here: http://elinux.org/BeagleBoard_....._.28RTC.29
It looks like there are plenty of RTC drivers right in the kernel source.
$ ls linux-3.2-rc3/drivers/rtc/rtc* | wc -l