Making a video loop in Poser 5 is certainly possible, but it involves some work with the math and variable nodes. You have to be careful, because Poser 5 doesn't always like it when you run out of video frames.

Step 1:

Load the background video.
File menu --> Import --> AVI Footage

Step 2:

Go to the Material Room and select Background.
In my version of Poser 5, the Movie shader node won't appear to be connected, but it works nonetheless. Chalk it up to one of those annoying quirks of P5. You can connect it manually, if you wish. It doesn't appear to matter.

Step 3:

Attach a Math_Functions node to the Movie node's Frame_Number channel. Set Math_Argument to Mod, Value_1 to 1.0, and Value_2 to the total frames in your background video. For instance, if your video has 35 frames, set Value_2 to 35.

Attach a Frame_Number node to Value_1.

The Movie shader has a tendency to "white-out" on frame zero, so you might want to add an additional Math_Functions node to prevent the frame value from dropping below 1.

Step 4:

Use the Firefly renderer.