It's not that complicated. In fact, it's one keystroke. Shift+N. As long as you can see the alpha (checkerboard) in the canvas window, it will retain the alpha in the freeze frame. You can also press shift+n in the viewer. I'm working in SD DV NTSC btw.
If you have a slug or something underneath, well, obviously it's going to kill the alpha.
And I've never had a problem with the black fringe. ALL alphas I do are straight alpha matte. No reason to do premultiplied unless you want it to also look good as a standalone piece. Premultiplied has lots of issues like the black fringe. This example is a double alpha luma matte, but an animation codec clip should work the same.
Attached is a screenshot taken right after pressing shift+N in the sequence.
