If nothing else, perhaps just make a life support block that works similarly to the gravity module. That is, you don't need to wear a helmet while attached to the ship after having activated the new block. Sure, it means you can wander around the exterior of your ship, but it does limit your non-helmet use to within certain boundaries.
I was also considering the idea of a two-block directional system. By activating the blocks in order you either activate or deactivate the life support. That way, entering a ship through an airlock could activate life support, and leaving would automatically deactivate it. This doesn't do anything to create an "enclosed space," however. You could set up the two blocks (or gates) without any walls and cheat the system.
Taking a page from Minecraft, you could look at the village generation for cues. In order for a villager to accept a home as habitable, it has to have a door and a roof block beyond it within a certain height range. You might enact a similar system to at least get some participation from players - such as stating that there must be a floor, walls, and ceiling beyond the life support block/gate to a particular depth in order for it to qualify for life support.
I was also considering the idea of a two-block directional system. By activating the blocks in order you either activate or deactivate the life support. That way, entering a ship through an airlock could activate life support, and leaving would automatically deactivate it. This doesn't do anything to create an "enclosed space," however. You could set up the two blocks (or gates) without any walls and cheat the system.
Taking a page from Minecraft, you could look at the village generation for cues. In order for a villager to accept a home as habitable, it has to have a door and a roof block beyond it within a certain height range. You might enact a similar system to at least get some participation from players - such as stating that there must be a floor, walls, and ceiling beyond the life support block/gate to a particular depth in order for it to qualify for life support.