Is it appropriate for junior officers to call senior officers by first name outside of work with no enlisted around?

Right on. That's how I feel about it as well.

This was a bit of a struggle at first for me as well. I shared an apartment wall with an E-2 when I was an O-1. Our spouses became friends, so by extension we started hanging out. I permitted the use of first name since we were in separate BNs and would not see each other. Then we went to Iraq in '03 and I was transferred into the same battery and he wasn't able to keep his mouth shut. My mistake.
From there, I had the pattern of first names with officers with whom I served at a lower rank, but only in close areas. With troops around, there were the standard courtesies including salutes. For my part, I never even thought about addressing my seniors by first name, in uniform or out. Using first names for officers (NOT enlisted, except maybe the 1SG if you're the company commander) is a judgement call. I did from time to time and usually during mentorship but not in front of enlisted soldiers.