The thread title includes the word "player"; so I am not thinking analogue input is required.
Using an SD card (or similar) makes getting the data onto it simple. Plug it into a USB card reader/writer, then write to it. Very easy with Linux.
Devices (including mass-storage devices and partitions on them) appear as files under /dev.
E.g., if you already have one hard drive, that'd appear as /dev/sda. Partitions on it appear as /dev/sda1, /dev/sda2 etc.
So an SD card in an attached USB reader/writer would (probably) appear as /dev/sdb.
Normally you'd partition it (creating one partition, /dev/sdb1), then format the partition, mount it to some mount-point (say, /mnt/sdcard) then stuff files on it.
But there's nothing stopping you opening /dev/sdb in a C (etc.) program then writing to it directly. No partitions, no filesystem; data can be written exactly how your player hardware requires it. (It's nice when an OS doesn't prevent you doing whatever you want to!)
So if I were you, I'd stick with some kind of memory card which can be plugged into a PC via an adapter or USB device; just FFmpeg and a simple little program would get video data onto the card without designing/building yet more hardware.
Just my two penn'orth...
Kat