Presumably you would use the TouchDesigner Spout In TOP, with Spout output from the video player.
I would start by investigating how you could use VLC or FFmpeg. Both of these would first need either an OpenGL context or a DirectX device which can be established with Spout functions.
Then you need an uncompressed RGBA pixel image for each frame of the video for the SendImage function. There is more involved of course but too much detail to cover here.
When I looked at building a VLC module it was too complex for me, but you might want to have a look at possibilities. Also, if Reaper uses a command line for for FFmpeg.exe, you might be able to modify that to use an output pipe.
I would then suggest using DirectX in favour of OpenGL with the latest DirectX methods in Beta 2.007. I can send you an example project using FFmpeg with a pipe if you find that there are possibilities. Just send me a private message here or use the email contact on GitHub.