WowAce

For guild members, the idea would be to display, optionally at logon, and in the roster, the last time someone logged in on any of their main or alt characters.

The roster displays how long it's been since someone logged in.  You would need something similar to a math.min(main.logintimeago, alt1, alt2, ... altn)

What do you mean by "display...in the roster"? The guild roster? The last online column is there currently and I think it is best to not modify the Blizzard windows. Or do you mean the main window of the addon? The one with the alts listed.

I think the only real issue with this idea is that I don't see a way to get the last online date. There is a call GetGuildRosterLastOnline() but it just returns the year, month, day, and hour-the same information show on the guild panel. You cannot get an absolute date. My attempts to calculate backwards from that data hasn't been successful. While I can get some what close, I cannot get it perfectly. Although I'll admit it is tough to test it properly since I need to find a character that hasn't logged on in over a month and one that I know exactly when they last logged on.