Do the late 2013 models (13" - (ME864, ME865, ME866), 15" - (ME293, ME294)) support RAM upgrades?
No. The RAM is soldered to the logic board in all of these models.
Here is the logic board from iFixit's Teardown of the low end 15'' model (2.0 GHz, 8 GB RAM, 256 SSD).
Inside the yellow box are 16 x 512 MB DDR3 SDRAM (8 GB total) memory chips. As you can see, they are soldered directly to the board and cannot be replaced without replacing the whole logicboard.
Per the teardown performed by iFixit, the RAM in Retina Displays is soldered to the logic board. It may be possible to undo the soldering and upgrade it, but the hassle is almost surely not worth it.