wildbear – I assume you’re saying that because the proposed pipe is attached to the external surface of a Lot’s external (common) wall, then it’s in the air space of that Lot (?).
If that interpretation is correct, then it’s wrong!
The fact is that once a pipe or indeed any item is attached to the common property (wall) such that the property would be in some way left damaged should that item be removed (in this instance by masonry fixings), then that item becomes part of that common property.
Whilst I don’t know how your Plan’s Special By-Law is written, it should for that very reason contain a provision that shifts the responsibility for the ongoing maintenance of repair of gas pipes so fitted from the Owners Corporation (O/C) to you and any subsequent Owners of your Lot.
Again, I assume that the O/C) is talking about an easement because the area directly below the common property (wall) is part of the Lot (?), and so any maintenance that may be necessary on the gas pipe would require that Lot Owner’s permission for access and to possibly position a ladder.
If that interpretation is correct, then the O/C is also wrong!
IF the Owner of the Lot ever became obstructionist you could apply for a Utility Access Order under the provisions of the Access to Neighbouring Land Act (2000), or alternatively, at a stretch the O/C could itself gain access to have such maintenance or repairs undertaken under the provisions of Sect.65 of the Strata Schemes Management Act (1996) provided it then invoiced the costs to you.
I hope this assists in some way with the lucky-dip that pretends to be Mediation.