The issue with arty spotting in WW1 was that the communications equipment simply wasn't up to the task. There were no man portable radios. The only radios were large, bulky units assigned to mid and higher range HQ's. Field Telephones worked, but they were also vunerable to having their wires blasted apart. Observation balloons and other elevated positions were certainly usable, and would have some form of telephone connection. Otherwise, you were forced to use runners.
The downside of all of that is it gave very little flexibility in calling down arty. You could do it off of a preplan for offensives. Likewise, the defenders in the trenchline could call down preregistered fire much of the time. That was about it.
What would often happen, is that if the attackers managed to take the first trench line, and even the second, they would be cut off from their own artillery. No wires across No Man's Land, no radio, so you're left with runners, which is far too slow (and unreliable) for effective artillery support. This is one of the reasons the standard german tactic was to let the attackers hit the first (and even second) trench line, and then counterattack and restore the position, when the Allies didn't have the ability to bring in their own defensive fire.
All of the above is of course, in reference to Indirect fire pieces. All sides still had a fair number of DF pieces (the French 75 generally, though it had sort of been modified for IF, would be a good example of this). In game terms, DF pieces are simple. Give them a 1-2 hex range, and they need LOS.
This is one of the reasons tanks were so useful initially. They became that heavy fire support on the other side of NML.
I'd agree balloon placement should give extra visibility. Aircraft flights, not so much so. Again, no radio, so the ability to bring down a barrage on time and target was iffy.
