Has anybody ever installed 1G pistons in a 2G engine?

So far no one has done this swap. The change is not attractive since 2G turbo pistons have a higher compression ratio (8.5:1) than stock 1G turbo pistons (7.8:1), meaning the 1G pistons will produce less power.

2G NT pistons, despite their higher compression ratio, are not generally considered suitable for a swap into a turbo engine because they were never designed to handle the stresses of turbocharged engines. This is the same argument that keeps people from swapping 1G NT pistons into 1G T engines.

Also, since 1G rods must be modified to accept 2G pistons, it is safe to assume that 2G rods would either have to be modified to accept 1G pistons, or replaced with 1G rods, making any such swap that much more complicated.

Anyone interested in such changeovers should know that Mitsubishi sells factory overbore pistons in sizes 0.5mm and 1.0mm larger than stock. Ben France gives the details here.