For those who play 3.X or Pathfinder, does metacreation function in Sigl? It blocks access to planar travel like astral spell don't function in Sigil. Summoning and calling spells don't work there either. However, teleportation seems to function in Sigil as long as you are teleporting within city limits, and spells/psionics that just require connecting to the Astral Plane seem to work. You can't just bring stuff in or bring them out. The description in the Expanded Psionics handbook says that metacreativity is not summoning or calling. Rather, the creatures and objects made by these powers are assembled on the plane the caster is on using material drawn from the Astral Plane. So I'm assuming that since stuff created by the metacreativity is specifically called out as not being summoning (and many of the powers are of the screation subschool) that they work in the City of Doors. Is that correct?
Psionics metacreativity and Sigil
I'd generally avoid any rules that make the game less fun for everyone. If you can justify letting people use their class powers in the City of Doors rather than wanting to leave the city as soon as possible because their character is nerfed there, justify away.
Nerfing class powers is never fun for anyone.
Therefore Astral Constructs should work, just as much as summoning should, or Teleportation powers or Warlock powers, or Binder Pacts or whatever. For abilities like that you are just going to have to find ways to creatively justify why they work there.
You're punishing the players if you restrict such powers from working in Sigil.
You could rule that the creatures and objects are created from material drawn from Sigil, giving them a unique appearance and flavour, without nerfing anyone or changing the rules.
Browser error...
I believe this is a fan concept and cannot recall where I got it originally, but this is how I handle it:
Sigil has it's own pocket Astral/Ethereal/Shadow planes. As long as you do not try to leave the confines of Sigil or bring anything into or out of The Cage things work normally.
This preserves the strictures of Sigil without needlessly penalizing the PC's.
This is how I would do it - so creations would be made out of decaying brick, smog, garbage, razor vine, etc. They would do what they were meant to, but would look and possibly act bizarrely.
That would be my take on it; an astral construct can be created in Sigil, while a summoning would fail.