Somehow missed this thread... was curious as to which direction Greg finally took.
Anyway, another alternative would be to setup Yagi's on both the house and observatory. I've made a few antenna's in the past (my first one was a Pringle can) and it is not difficult. Google is your friend here... make sure to include DIY.
Search for Yagi or Cantenna.
Commercially made Yagi's are readily available. These can throw a signal many kilometers. My Pringle's can could just touch half a mile, albeit with a dubious signal. You'll need solid mounts if you have high winds in your area.
Note: the one's I've built were for the 2.4Ghz range (802.11b/g/n). I've never built one for 5Ghz (802.11ac).
Last note: Ethernet is still your best option as the throughput is 1Gbps if your router's LAN ports supports 10/100/1000 (see discussion here)
OIC!