The almanac is valid for a long period, it's the ephemeris data (which adds minor corrections to the almanac data) which only has a short life expectancy. See e.g.: http://gpsinformation.net/main/almanac.txt

I imagine the GPS subsystem would cache the almanac data anyway until it becomes invalid, unfortunately we can't check this.

Afaik it's possible to generate long term orbit data, but you'd still need to feed this to the chipset. So yes, afaiu theoretically you could provide your own ephemeris data, but you'd need to do some serious number crunching every time you start up, and you'd also need to know how the format the data to pass it across to the GPS chipset. See e.g.: http://www.gpsworld.com/wireless/per...ssistance-3731