I think it's possible for the S2 due to its slightly larger battery. Apple might be able to curb some battery issues through software such as limiting the always on display to certain watch faces that consume less battery and dimming the display.
Considering that Apple took a very conservative approach to the original WatchOS compared to WatchOS 2/3, i think Apple is somewhat conservative with the S2 battery life generally speaking.
I wouldn't be too surprised if Apple introduced an Always-on display for the S2 in the next major update. I too am able to go to bed with about 50% battery life on my S2, so an Always-on display may barely make it through a work day assuming GPS isn't used.
Apple will just need to remind users that the 18 hour battery life excludes the use of an Always-on display. They already do that with GPS (rated at 5 hours iirc)
The main issue I can foresee is the vast number of people who will complain about the battery life if Apple did implement it on S2... coz people generally find lots of things to complain about if its Apple. So I think Apple is more likely to implement this in future versions of the AW once they've figured out how to squeeze in more battery power in such a small package.