I've just stumbled on fbcp and tested it: works like a charm with the Adafruit modified Raspian Jessie-based PiTFT 3.5" Resistive Image for Pi 1 and Pi 2 (September 24, 2015):
Update I found another option which doesn't require fbcp: mplayer though SDL. Although the guide is for the 2.8" display, it still applies for the 3.5" too. One thing to bare in mind: from my experience so far it's impossible to playback smooth video without frame tearing on the 3.5" screen though. (If you have a workaround, I'd be more than happy to update this).
For reference (in case the link changes at some point), here's the mplayer option:
First install mplayer:
sudo apt-get install mplayer
Then pass use the Adafruit TFT frame buffer (
sudo SDL_VIDEODRIVER=fbcon SDL_FBDEV=/dev/fb1 mplayer -vo sdl -framedrop path/to/your_video.file