Decoupling Capacitor
If you're still in layout, then you can go back to the schematic and fix the circuit.  You may have to then move parts around to fit the new cap, depending on how tight the congestion is in that area.  Still, fixing it now is far better than applying a kludge later.

Besides, this is one of the things that should be looked for in any design review, and checking this is also something that should fall out of normal layout procedures.  When you go to lay out a part, the first thing you do is look at the power and ground connections.  If you see pins connected to power that don't have a bypass cap, you go back to the schematic and check whether these are power pins or just signal pins permanently tied high.

It's hard to imagine how a missing decoupling cap would go undetected all the way thru layout.  This therefore a non-question.  If you're thinking about decoupling caps in layout, then you'll fix problems there.  Conciously putting in a kludge around missing decoupling caps in layout makes no sense whatsoever.

If you do manage to screw up and miss a decoupling cap, the question then becomes how to do the rework.  That depends a lot on the geometry of the part.  Most of the time soldering a small leaded cap directly between the power and ground pins of the package will be the simplest fix, but this will vary case by case.