I think it could be feasable. There is a mod for mincecraft \"Gelacticraft\" which does just that for things like moon bases. The moon has no air so you have to make sure your base is fully enclosed, have the air generation system, power system and air supply systems setup to pressurize your base. It does not seam to create any additional noteacable lag on a weak minecraft server unlike other mods. So I think it could be very doable if done right.
Would probably involve multiple blocks, such as an Air Generation block which produces X amount of air per time period. Oxygen storage tanks which would be simmilar to the energy storage blocks, its a block that stores Y amount of oxygen. Oxygen vent blocks which are used to pressurized/depressurize a section of the ship. A small ship may only need 1 vent block while a larger ship might need several to cover the appropriate area. For airlocks, thats easy, no need for a special type of block there, just a small room with 2 doors and maybe a vent block. 1 plexdoor lead sinto the inner area of the ship, 1 to the outside. For the pressure seal, we aren\'t looking for minute holes, just basicaly the vent block checks its area for any 1x1 or larger holes leading into open space. The vent block could also have a maximum range it checks, say 20 blocks out. If it doesn\'t encounter the pressurized capable area of another vent block or a solid wall, it assumes a hole and is unable to pressurize its area. So you could have 2 areas which ar epressure capable, but otherwise 2 large to pressurize by 1 block, but 2 working together are able to pressurize the area.
The fun then becomes when you get holes punched in your ship due to battle and suddenly loose pressure as a result. Would add a new dynamic to ship building thats for sure.
Could add an emergency bulkhead door block which is normally open but can be set to close if pressure integrity is lost in a section. For example in my above example of 2 pressure capable areas, nomally the 2 ventblocks working together can handle it with the bulkhead in between. With the bulkhead closed, now its 2 independant close doff areas which the single vent block in eahc can handle. So your goign around, you have your large open area with its 2 vent blocks and all is fine. Now you get a hole punched in the wall of 1 of the areas. Now that area is nolonger pressure capable and looses its pressure, the 2nd area would also become depressurized as a result, oh but we have that bulkhead door there and it slams closed automatically seperating the 2 areas allowing the undamaged area to remain pressurized. Like any door the bulkhead door could be opened/closed manually by the player just like a normal plexdoor. It just has that extra feature that it closes automatically on a pressure change on either side of the door.
