While there are players that can use an USB memory stick, the fact is that with the move to larger GB does mean that a player may not understand a 64GB stick so my answer is no to the question as you wrote "any USB Flash Drive".
There are portable MP3 Players that can use sticks and cards so that's not the question.
Is it possible to use any USB Flash Drive on a portable MP3 Player.
The Creative 'Muvo' player comes in two parts, the player and the USB Stick, but we can only use that stick because of the way it is built. Is there any players that will allow any stick to be used?