Combining shadowmeld and true sight?

I am (also) working on a more "stealthy" hero arena map, and one of the key concepts is that all heroes are invisible when standing still, using the shadowmeld ability. But to avoid the obvious stalemate when one player hides and does nothing, or goes afk, I would give all heroes true sight when standing still as well. This is slightly harder.

There does not seem to be an event for Unit Is Stationary, so all I can do is remove the true sight ability when the unit does something, not add it when it does nothing. The glass is still half empty until someone tells me how to detect when a unit is not obeying any orders.
^_^

There is no straightforward way to detect when a unit is shadowmelded, but there are some trickier ways using periodic triggers.
The coolest I can think of is to use an unused player slot, not sharing vision with other players and with full vision of the map. Then check if the hero is visible to this player, if it is invisible then it is probably shadowmelding, the other possibilities are sorc invisible and windwalking, you can use order comparisons to tell if an invisible unit is moving or not, because invisible units wont automatically attack/autocast.

Will this work anywhere on the map, or not behind cliffs, mountains, trees, vision blockers and so on?

I think it would work everywhere because it is in his VISION (which is everything on the map in this "player's" case).